Say a creature owns multiple items with wording similar to this:
As an Action, you may speak the command word to [...]
Say the creature has deliberately obtained such items, perhaps by creating them itself, to have the same command word.
Can this creature use one action to speak the shared command word and activate multiple items at once?
The purpose of this question is not to cheese my DM. I am writing a short story and like to keep my D&D fiction as close to rules-accurate as possible.
Take a look at the text for each magical item that requires a command word. For example, here is part of the text for the bowl of commanding water elementals:
While this bowl is filled with water, you can use an action to speak the bowl’s command word and summon a water elemental, as if you had cast the conjure elemental spell.
Note how it says you use an action to speak the bowl's command word? That is fairly specific in its meaning: you are using your action to speak the command word for that particular item.
If it were just a matter of uttering the command word in general, you'd be able to activate the item as a free action (since speaking is a free action). You have to have an intent to activate that particular item and you are using your action to do so. You can't do more than one thing with a single action.
As long as each item requires both a command word and an action to activate, you cannot activate more than one at a time. The requirement of the command word isn't really even relevant, as it's only real function is to restrict someone from using an item they don't know the command word for, and stops you from picking up and using the wand that that wizard just dropped.
An action used to activate an item cannot be shared across multiple items. Each item requires it's own action expenditure to activate. You only have a single action per turn, and that action cannot be used to do multiple things, even if those things are similar or closely related.
That being said, if the items require no use of an action or bonus action to activate, then there is no restriction on activating as many of them as you want at once (up to how many you can reasonably hold or wear at once, you only have two hands after all). The number of items that don't require an action or bonus action to be expended to activate them, however, is rather small.
What the rules do say about command words is as follows (DMG page 141 "Command Word"):
A command word is a word or phrase that must be spoken audibly for the item to operate. A magic item that requires the user to speak a command word can't be activated in the area of any effect that prevents sound, such as the area created by the silence spell.
This is under the "Activating an Item" section which states:
Activating some magic items requires a user to do something in particular, such as holding the item and uttering a command word, reading the item if it is a scroll, or drinking it if it is a potion. The description of each item category or individual item details how an item is activated. [...]
These rules don't really tell us, well, anything about what happens with multiple items having the same command word. As such, it would be up to the GM what happens in this case. Perhaps worth noting is that the section is called "Activating an Item" and not "Activating Items" but that seems like too small a detail to really pay any mind.
Notably, there are also at least somewhat similar things that can already be done, such as setting up multiple glyphs of warding to activate on the same command word. This doesn't really say anything about Magic items, but it certainly makes activating multiple at once less unprecedented.
Theoretically, you could make the items not require an action at all and only require speaking. While this would certainly change the items compared to their appearance in the books, if they're already being custom-crafted or collected it isn't such a drastic change. I mention this as an option because it isn't unheard of in fifth edition; an example of such an item is the Pole of Angling which has been discussed in the following:
For that matter, you can honestly change anything since you're the writer and can take creative liberties. It may not be the most accurate to change the action-economy requirements or to have multiple magic items activate at once, but you, of course, have the freedom to do whatever you'd like.
