I am thinking of using a powerful panasonc (240CFM or more) inline fan up in the attic to exhaust both bathrooms through the same hole in the gable wall in the attic. I'll use a "wye" connector to run two 4" ducts into the fan. Does this sound feasible?
I don't know if your done with the fan installation yet, but I installed a inline FanTech 100cfm with flex duct, ran it 20ft to a gable end wall for a 6'x10' bathroom, works fine.
I talked to FanTech they told me the fan is rated for 100' & would be fine running 20'.
I can hardly hear this fan running.
If you go with to much cfm it could draw to much air (back draft) thru other sources like the furnace & hot water heater causing a pollution problem.
It will also draw air from every crack & crevice in the homes outer shell thats not sealed.