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?
Does the temporary hp from the protectors stack or not?