Bird box question

I have an old utensils box I made a while ago and now it's not needed I wondered if I can repurpose it as a double bird box .
After closing off the open side, will it be better with holes next to each other (semi detached) or on either end (back to back)? Do birds even nest next to each other?

 

Also, if I made two different sized holes will two types of birds ever nest next to each other?

 

Never mind, I've stuck a slate roof on and drilled a couple of 30mm hole in different faces.