Can bat houses be mounted right under martin houses?
Yes. In fact, bats commonly roost close to birds and don’t seem to mind martins. We actually have a BLUE BIRD BAT HOUSE which can comfortable house both blue birds and bats and the two species don’t mind each other. At this point we’ve not seen evidence to suggest bats and martins will share the same house but no doubt they can exist in different houses even if mounted on the same pole.
Blue Bird Bat House: http://www.bats-house.com/blue-bird-bat-house