Co-habiting?

I remain bemused.

I often catch sight of birds flitting to & fro from the nest box outside our kitchen window. Most years a blue tit pair has taken residence & brought up a young family.

This year two species of birds are making regular trips, a blue tit & a house sparrow.

What’s happening here? Can we expect to see young blue sparrows or house tits? Or are they just co-habiting, with two nests in the one box? I would have thought that was a squeeze.

I look on with interest.