bird box plans

A year or two ago i had a sparrow terrace against the rear wall of my house,   which hadn't been occupied so i decided to take it down.   the box had a small gap behind,  perhaps a centimetre or so due to the nature of how it was built.     As I took it off the wall i found there was a bat which was roosting behind the box (i wasn't able to reattach the box but did put a bat box in its place which is sometimes used by bats now).

However this has just got me thinking,   i'm going to build another bird box,  not sure what type yet,  but was wondering whether i should build in a small gap at the rear so that in addition to it being a bird box,  there is space behind for a bat to roost.

any reason why I shouldn't do this?   The bat and bird would be physically separated by 18mm of wood and have a different access point.