Most probably share one with each other . I remember that in the starting chapters Nagi once mentioned it .
They do. And kotone has imagining him getting sandwiches and incest relationship lolol. I think it's when they're remarried after kotone died twice
They would have to share it. Kotone had asked and Nagi (I think) said he was glad since he wouldn't have to share Kotone w/ somebody else.
My theory is, since haru came first
If hao and nagi still together, both of them will sare haru haha
*share
I wonder if Hao and Nagi never got seperated, would they share a chosen one or would they each get their own chosen ones still?