Roosting or nesting

I have a camera box that’s been used by a roosting blue tit since November/December. The same bird has come back every night since arriving sundown and leaving at dawn. 
We had hoped that it would nest there even though we didn’t know if it was male or female. 
Anyway, two days ago a female showed up in the box giving it a good look, we know it was a female because almost immediately it began nest building,  after two days the nest is well on its way. 
The problem is that the long term roosting tit came back last night and tonight and appeared agitated. I’ve a feeling it’s not either of the pair nesting and eventually it and the female will meet. 
what’s the likelihood of the nesting male being the roosting bird or would it stay away from the nest? Thanks for any ideas