That is not how Tentacles works. Each Kraken will trigger, but they only do it once per board state, regardless of how many 4- or 5-matches appear. Once matches gems disappear and new ones fall into place, Tentacles can fire again—on each Kraken. Just tested this to make sure.
This suggests that Megavore works the same way. If you have two of ’em, there is a 15.36% chance that at least one will fire on any given 4+ match. And if you were using 4, there would be a 28.36% chance of an instagib. I don’t want to spend the resources needed to acquire multiple copies of Megavore to test, so I will let someone
richer crazier else test this for me. Or wait for confirmation from someone with a gem next to their user name.