Nuthatch nesting behaviour

Hi all, we are lucky to have nuthatch’s in our nest box this year. They have been taking mud to it and tapping away. Now we see food being taken in too.

We also have a new bird box on the same wall of the house but about 15 feet away. We had been pleased to see blue tits apparently nesting there too.  A couple of days ago it seemed a nuthatch was attacking the blue tit at the box. Now we’ve heard the same tapping, and seen a nuthatch fly out of the blue tit box. Is this normal behaviour to take over a nest box or to nest so close to another pair?  I’m assuming it is another pair of nuthatch as the first box is still in use.

Hoping someone can help answer our question.