Personally, when my SO & I first did overnights, we scheduled them so that we both had overnights the first few times. I feel it was a great way to transition. All parties knew the situation. Perhaps we were lucky that we had that opportunity. It definitely allowed us to more gently ease into individual overnights rather than jumping into the deep end where one of us is alone all night.
Me: 55 straight male in a W
-Wife/Partner: C - poly female - together 4 years
--Her BF T: poly male
My GF: M - mono? female