Not sure about favia vs favia as there are tons of species of favias and I don't know the difference. But acan vs acan if they are in the same species it should be ok. I have acan lords touching each other with no ill effects.
Well in my experience, which granted is limited due to me not letting corals touch
Acan Lord vs Acan Lord (nothing happens, they're bros after all!)
Acan Lord vs Acan Ech/Subech (Lords lose)
Favia (underoos) vs Open brain (favia loses)
Thanks! Mostly I was wondering if I could glue multiple favias on a single rock and they would eventually grow into each other. But looks like this wont work.