I don't know enough about codes to sight a specific reference that would or would not allow such an instillation. But here's a few things about code I do know to consider.
1. Mechanical codes require that all exhaust ventalate in a manor that doesn't create a nusance. IMHO, if one bathroom exhause is partially venting to another bathroom, that's a nusance. So any dampers used would need to be air tight.
2. Codes require that manufacturer's instructions be followed. So unless the instruction manual includes directions for combining exhause, a building inspector could turn it down for not following manufacturer's instructions (and the few instructions I've seen for exhaust fans always show a dedicated vent).
Originally Posted by Bertha
Another item, I'm assuming that only one bath fan of the 2 are being used at a time.
Why can you make this assuption. Are you going to make it impossible for both bathrooms to be used at the same time? It would seem that if the manufacturer requires a 4" exhaust, that if you are going to combine two exhaust lines into one, you would need to also upsize the exhaust at the "Y" to 6" (a 6" exhaust is more than twice the size of a 4").