I never played Resistance 2, actually... I beat the first and just, well, never got around to the second. Now that there's a third one I better get to it!
Alrighty, after messing around with a bunch of things, I think I found the problem. (Emphasis on the word THINK, because like I said, I'm new here too, and I certainly don't know my way around Construct as well as some of the other guys.
But, I THINK the problem is that the game is treating both enemies as a single object. You should find some way to use families or containers to group the gun and the tank together, too -- on the second layout, see how when you blow up one tank, the other tank's gun vanishes? It isn't distinguishing between them properly, which might be leading to this problem. I haven't worked with that stuff yet, so I can't really help any more than that... hope it's enough to get you going. If it isn't, sorry!