Very good question AitchBEE and one I have wondered about myself. I have two of them quite close together and I find that only one is ever occupied at a particulr time by either Blue or Great Tits. It used to be just Blue Tits but the last two years Great Tits have also moved in as the opening has got slightly bigger over time. I have never had both occupied at the same time but whether this is because of territorial rights I have no idea as they are normally quite friendly birds. I know you will only get 1 Robin to a garden as they will fight to the death to keep it that way.
OnePlus 5T review: Hands-on