Why can't I revive my Co-Op partners troops? I can use other skills on them. Why not revive?

You can use it, I have done this. Sometimes the skills are a bit fiddly in co-op, same with divine interventions.

With DIs, the trick to using them is to left-click the target squad (selecting it), then target the precaution at the portrait of the squad near the bottom-mid. I personally didn't have problems with rez, and therefore haven't tried how to get it to work, but I'd imagine you can get it to cast somehow.

Happy casting,
Melor

Edit: Pre-what? To answer with a Cerebus quote: "Most holy is INFALLIBLE!" (I would need some icicles on the last word though).

By "precautions" I assume you mean Divine Interventions.

select revive click on co op squad