Do multiple Boost Shields actions stack with each other (to increase the defense in one zone by more than 1)?

On one hand, I could see this working just fine from a balance perspective. You're spending an action, performing a Hard check, and causing the ship to suffer 1 system strain for 1 extra setback die on one targeted defense zone.

But also for consideration is, how many people are gonna have access to the shield systems? Maybe two people max on a freighter? A mechanic in the engineering shaft and an astromech plugged into the cockpit's scomp link, maybe.

At any rate, it should be fine. System strain is a limited resource, as are player actions. And if you've got the players that have the high Intellect/Mechanics ratings, I say let them do what they were built to do.

Until a Despair gets rolled and a coolant pipe gets ruptured in engineering

Total opinion of RAI: In the description for the boost shields action, it states that "Additional [successes] increase the duration by one round per [success]." While this is for the result of a single check, I would rule that any additional successes granted by additional checks are treated in the same manner and could extend the duration rather than increase the defense of a zone. I don't think the rule is intended to allow more than one increase to the one zone. There is only so much that you could boost the system in my opinion.