
Wingblaze |

By "stack" do you mean "install multiple force fields somehow, and then run one after the other"? I'd say that's fine.
If you mean "run them all at once", that's trickier. They're kind of like temporary hit points which don't really stack. That said, getting them all turned on (at a standard action each) might be tricky. I think I'd let someone turn them all on, but all damage had to go to one first (say, the most powerful one) then when it collapsed, the next one.
But I'm really not sure how you can get around the restriction unless you're talking about a force field armor mod combined with a mechanic trick energy shield.