It's viable under RAW ...
... as discussed in this Q&A. Beyond that, any ruling on how to avoid the attack falls into a host of situationally dependent details. It's difficult to arrive at a general answer that fits all situations.
Turn order can influence the success of this tactic
One significant issue will be the initiative score of the two artificers. If their initiative scores are not identical, or not sequential, there is a turn-based "delay" in a given round as the telekenesis step is awaited in the turn order after the toss step. If the creature they want to send on holiday to the Astral plane has a reaction, movement, or an action before that telekenisis step, they can probably "see and avoid."
Whether or not they realize the nature of the threat you can emulate as an Intelligence Ability check, if necessary as a DM.
Your use of the Readied action accounts for most of the above problems.
Legendary Actions might interrupt this
Creatures with legendary actions pose a case where, depending on what the legendary action is (functionally, a legendary action works like a reaction taken on another creature's turn).
A legendary creature can take a certain number of special
actions—called legendary actions—outside its turn. Only one legendary
action option can be used at a time and only at the end of another
creature’s turn. {snip} If surprised, it can’t use them until after
its first turn in the combat.
If your party surprises the creature, I think you've got 'em! 😉👍
But if you haven't surprised them, you run into a turn based mechanics problem of "when is my readied action going off versus when did the bags land there versus when did that reaction occur?" This needs a DM ruling.
If the telekenesis reaction isn't before the other artificer's turn is over, who is to say when it happened in terms of 'when did the bags land near their target?'
If the telekenesor interrupts his ally with his reaction, the toss never goes off, or does it? DM ruling needed. Even then, if I have a an adult dragon with a Wing Attack, that has this effect as a legendary action ...
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
It can move out of the way, and the wind of that force can arguably move the bags. Likewise, but to a lesser extend, with the tail attack. If the dragon chooses to attack the pair of bag rather than the pair of artificers.
The dragon makes a tail attack.{snip} Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.
While it doesn't say explicitly that it knocks small loose objects any distance, one can easily rule that it does and it makes sense from the PoV of verismilitude.
More generally, any legendary action that includes movement or an ability to move the pair of bags could potetially frustrate such an attempt. The DM has to rule "whose reaction goes first?" in a case like this - the other artificer, or the targeted creature? I'd be inclined to use either Dexterity or Wisdom Contest (Basic Rules, Chapter 7, p. 61) to adjudicate that as a DM.
Both participants in a contest make ability checks appropriate to
their efforts. They apply all appropriate bonuses and penalties, but
instead of comparing the total to a DC, they compare the totals of
their two checks. The participant with the higher check total wins the
contest. That character or monster either succeeds at the action or
prevents the other one from succeeding.
The creature's allies may likewise have an action available, or a reaction, that interrupts the two artificers getting their "lobbed pair" + "telekenesis" sequence to occur before any interruption of the sequence. But to thoroughly frustrate your tactic, they'd have to be readying an action to "throw back whaterver is thrown at our boss" or have an ability similar to the Monk's Deflect Missiles (uses a reaction). Otherwise, your use of readied action should prevail.
Again, a Dexterity Contest is probably the best way to resolve "whose reaction comes first?" if one is used as a counter to this tactic.
If the target has a familir, like an imp, the familiar may be able to do a cinematic "toss the grenade back!," if it is able to use a reaction or has an action readied. You (as a DM) could call it a "help" action or an "interaction with an object" in terms of how that fits into what familiars can do. I'd still call for a contest as DM.
Is a Dexterity save against this RAW?
No, since it's not a spell attack. The bags are thrown like a flask of oil or alchemist's fire, it seems, or holy water, but the target to hit is "anywhere within 10' of the target creature." That's not a very high armor class. I'd estimate an Armor Class of 10 to start with, and the bags are an improvised weapon. That leaves a chance of failure in terms of "We threw it, but missed by throwing too far or not far enough!" But it's not beyond reason to rule the Armor Class as lower than 10. This leave us with this DM ruling again: What is a reasonable armor class for a CEP of 10'? (CEP = Circular Error Probabaility, a measure or accuracy/precision on bombs dropped or mortar rounds lobbed ....)
As an aside:
I love it when people come up with clever little tactics like this. :)
