A lot depends on whether or not you've "rolled for initiative".
Not that the roll-for-initiative procedure itself actually exists in Dungeon World, but since you've got history with D&D, you've almost certainly experienced the mode switch in play that happens when you roll for initiative, right? Before it happens there's just random dungeon goofin' going on, it's not particularly important where everyone is, and anyone can do anything at any time. After it happens, life and death are on the line, it can be very important exactly where everyone is, and everybody's share of actions is handed out in turn.
In broader terms, the freedom characters have to act and interact with each other is limited by the pressure of the situation they're currently in, and Dungeon World provides some tools to impose limitations on your players that line up with the scene pressure.
The GM's Pressure Switches
Just to explicitly lay out the structure that's underneath this, you have core GM moves that express the pressure of situations: separate them, put someone in a spot, tell them the requirements or consequences and then ask. It is completely within your powers to at any time single someone out of the group, demand they alone deal with any problem you wish, and impose whatever conditions you'd like on anyone else who wants to deal with it instead. (Or, slightly more broadly, you the GM are the mediator of the at-table conversation. Regardless of how you and the players hold and participate in your group conversation, to the degree that it affects the game, you have final say on who you want to listen to and whose input will matter.)
However, it's also in your GM principles to make a move that follows. Anything you're doing should be framed not as GM caprice but as a reasonable consequence of the fiction that's going on. In this case, it means that you should use a level of pressure that fits the situtation. And most often, the pressure in your story is going to come from time, and how you don't have as much of it as you'd like.
I'm The Map: Distance As Implied Time
Dungeon World is not a game that demands a tactical map of you, or that operates with the precision a tactical map demands. However, an abstract map (or, let's be honest, a tactical map where you stop caring about the grid lines) is still a very useful tool for running it, for helping players visualize the battlefield, the various threats, and their own positioning.
If you have and use an abstract battle map, you can shift the focus over to Clericsdottir in the middle of a fight and present her with the choice to heal Wizzrobe, who just got smacked by the ogre, or stop a goblin from running to hit an alarm gong, and the battle map will help reinforce the time pressure anybody else would operate under to do either of those things. Fightgar already knows he's stuck in with the ogre and Shanksworth already knows he's getting ready to backstab the shaman based on what the battle map says, so they don't have this expectation that they can just cut in front of the running goblin.
But you don't need a battle to present someone with a map-based reason why they can't get involved with something. Even a simple town map can present this sort of pressure. Suppose Shanksworth is off trying to haggle a couple dozen more coin out of the latest big gem you pried out of a statue eye, and Clericsdottir is doing the divinely needful across town at a shrine. If you have a fellow member of Clericsdottir's order pull her aside to have a conversation about Shanksworth, the distance between the two PCs is enough of an excuse for Shanksworth not to be able to get involved in that conversation as it's happening.
Repent, Harlequin: Clocks As Defined Time
Dungeon World doesn't talk about clocks much - they feature much more heavily in its progenitor Apocalypse World and get some screen time in some third-party Dungeon World supplements like Planarch Codex. Clocks are basically narrative hit points: the Lost Treasury of the Dwarven Kings tips closer to the chasm's edge, its integrity dropping to 2/8. They're usually visualized as slices partitioned out of a circle, like slices of time in the 12 hours of an analog clock face.
Why can't Shanksworth just try to pick open every last chest in the treasury? Because he hasn't got the time. So a situation which might seem laughable in normal play - Clericsdottir, you've got to get this treasure chest open, no, Shanksworth can't help because reasons - suddenly makes sense when everyone's on the clock.
Collaboration: Planned and Ad-Hoc
In Dungeon World the pressure state isn't a binary "under initiative" or "not under initiative". It reflects how high-pressure the plot currently seems, and that's an analog judgement. However, there isn't exactly an endless, seamless continuum of game mechanics you can use when one PC wants to help another one out, so here's three touch points for you, in rough order of how close somebody needs to be to the current focus of the conversation, the "spotlight", to use.
Making most moves, such as Defend. Yes, even Defend is supposed to be an impactful moment -- tightening your guard to take a blow has the same narrative weight as swinging for the fences. Most moves only come out when a player is the singular focus of the spotlight, or when the pressure is low enough that the spotlight is wide enough to fit anybody who happens to speak up.
Making the Aid move in particular. In original Apocalypse World, Aid is explicitly set up to be a move you roll in order to support someone else rolling some other move - you make your Aid roll at the same time as their action. It's really no different in Dungeon World - if somebody's in what seems like a sensible position to help, then instead of letting them take over the spotlight or roll the same move as the spotlight character, you can let them roll Aid to represent their help.
Spending hold and bonuses from other moves, such as Defend. Or the +1 forward from an appropriately discerned reality, or passing off information Spout Lore passed on to you. Anything a player has from a move is theirs to hold onto and they don't need much of an excuse to use it, though keep in mind the riders that come with it. Discern Realities is explicitly a follow-up on an answer you learned; hold from Defend only works as long as you're still defending the thing you dramatically posed in front of before, and "you'll lose hold from Defend" is certainly a consequence the GM can ask about.
To make one of these more restrictive, you can attach a price to pay - taking damage or -1 forward, giving something up, throwing a Defy Danger to use it properly. To make one of these more permissive, you can alleviate the downside of a move, or with the freedom you have on a 6-, allow the move to essentially succeed but at worse consequences.
But that's all a rubric for making a judgement call in the moment. If you find yourself either deliberately or accidentally wandering into a proper high-pressure scenario, you might want to consider actually implementing "combat rounds", in a way that Apocalypse World has suggested explicitly for that highest-pressure situation of PC on PC combat.
First, hear from everybody what their plans are, then group those plans into blocks of intent where the people within can easily aid each other, then resolve everybody's immediate intent, shifting focus as dramatic and necessary and leaving people when things start to become uncertain for them, and once everybody's original intent has been addressed, start from the top again. Everybody gets to see the same number of intents through to the end, which is a pretty fair go for a universal framework.
A Final Caution: Overpressure and Underpressure
There are two especially common mistakes that a lot of people make on their first tries at running combat in Dungeon World. I'm not saying I see any particular evidence of either in your post, but as you're feeling out appropriate responses in a combat scenario you should keep both these harmful extremes in mind.
Overpressure is when you make things more dangerous than the fiction says they should be. This can often be caused by a lingering feeling that "the monsters should have a turn too", that you should follow up any nonlethal blow Fightgar lands with Hack and Slash with a "monster move" that "requires" Fightgar to Defy Danger in order to stay save, even if Fightgar chose to just take a clean hit on a 10+. But Hack and Slash is structured to answer the entire question of if you hit or get hit in melee; if you don't get hit then you don't, and if you do then that's just some straight-up damage or other bad effect, not something Fightgar needs to respond to. Over-requiring Defy Danger will make combats mechanically dramatic, which can cause real problems with dissonance and boredom when the combat itself isn't set up to be fictionally dramatic.
Underpressure is when you make things less dangerous than the fiction says they should be. This can often be caused by expecting that "the monsters will have a turn too" and not taking sufficient initiative as the GM to present the threats and obstacles that the monsters should sensibly present, but rather waiting for an explicit 6- or golden opportunity from the players that may never come. Underpressuring your conflicts will leave your players with a lot of freedom to act, but simultaneously not actually suggest many useful vectors for them to act on other than the one that they know works - when the monsters' HP reaches 0, they lose.
