Venting Two Bathroom Fans to Single Roof Vent
on/off at the same time

Problem ?: NO.
No problem with having two fixtures on the same switch. As long as the combined amp ratings of the fans (and anything else on the same circuit) is below the amp rating  of the circuit breaker.

both ducting to a y-adapter attached to a single roof vent opening

Problem ?: Maybe.
Maybe. When bathroom fans are controlled by separate switches it is not allowed to have them share the same duct because one fan will push the humid air from one bathroom to the other bathroom. Will that be the case with both fans on at the same time? All things being equal, fans CFM's, length of ducts, size of ducts it may not be a &quot;problem&quot;.
With different size ducts and different fans i can see one causing the other to not work efficiently. (This is speculation on my part, an HVAC tech is the proper person to ask.)
My gut feeling is it is not a good idea.