Why is it not a good idea... batrooms are back to back.. both hoses
take air out of bathroom.. go into a fan unit in the attic and it blows
the air out the top of the roof.. not understanding the problme with
that.. what am I missing. .


I don't know for sure, it's just on my list of things not to do.

I think it has to do with creating a connection between the
bathrooms, allowing noise and odors to travel between them,
and letting one bathroom, (the one with the open door, and
hence low resistance) provide all the air the fan can draw,
so that the other (with the stench, closed door, and high
resistance) keeps it's oderiferous gasses, but that's all guesswork.

--Goedjn