If an artillerist-artificer has 2 protectors, does the temporary hp given by the protectors stack?

Question

This is a somewhat loaded question that I could have stated more neutrally, but I didn't.

Premise: the artillerist has a choice of 'cannons' to use. One of those choices is the protector which does the following:

The cannon emits a burst of positive energy that grants itself and each creature of your choice within 10 feet of it a number of temporary hit points equal to 1d8 + your Intelligence modifier (minimum of +1).

At level 15 this artificer gets to use two cannons. The feature reads:

You can now have two cannons at the same time. You can create two with the same action (but not the same spell slot), and you can activate both of them with the same bonus action. You determine whether the cannons are identical to each other or different.

So you can have two protectors out which activate at the same time, but does their activation count as one iteration of an effect or ability, or two? This is important because temporary hp can't be added together. If the cannons are adding two separate pools of temporary hp they can't be added together, so are the cannons effects considered together or separate?

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Does the temporary hp from the protectors stack or not?

Answer 1

They do not stack

On each of your turns, you can take a bonus action to cause the cannon to activate if you are within 60 feet of it.

Your situation is such that you are using a single Bonus Action to issue a command to multiple creatures who then act accordingly. In this case, both cannons respond to your Bonus Action command to activate their effect, which is to hand out 1d8+Int modifier temporary hit points to creatures within their radius.

Your use of 1 Bonus Action is strictly to issue the command, it is the two cannons themselves that are creating the effect per the command. The fact that the effect occurs simultaneously (or nearly simultaneously) does not discount that they are two effects. As such, the standard rules for temporary hit points are applicable and thus they do not stack.

https://www.dndbeyond.com/sources/basic-rules/combat#TemporaryHitPoints

If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones.

The net effect of this is that those within the radii of both cannons will, on average, have more temporary hit points since they can consistently pick the better d8 roll.

Answer 2

Temporary HP never stack.

You've already mentioned it, but here's the rule:

Healing can't restore temporary hit points, and they can't be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones.

It doesn't matter whether this counts as "one iteration of an effect or ability, or two". Temporary hit points can't be added together. The two protectors both attempt to grant you 10 temporary HP, they both succeed, and you now have 10 temporary HP.

In this case, you do get to roll the d8 twice and keep the higher number, so it's slightly better than just having the one protector.