Multiplying bathroom fans?

Okay, so while I just happened to be strolling through the space above my ceiling [yeah, right] I happened to notice the bathroom fans didn’t have any ducting leading from the fan units to the roof. They just blow straight into the ‘attic’ space. So after seeing a bunch of articles that says that violates construction codes and is a rather bad idea in general, I looked again and I think I figured out the reason: There’s a vent in the roof above one bathroom fan and not one above the other. Apparently there was only ever one vent-hole made for the bathroom(s).

The actual problem with that is something I discovered while doing research on ducting. It’s ALSO a violation of code (and a bad idea) to put a Wye ([FONT=Tahoma]y-shaped joining piece) along ducting for the two bathrooms. The latter is because fumes pulled from one bathroom can, conceivably, get pushed into the other bathroom.

Ah, but there is a solution: There are actually fans available that are designed to mount right under the roof vent and pull air from multiple ducts. Flip the switch in either bathroom and the multiple-intake fan turns on. The effective result is that anything pulled through the ducts will immediately get pushed outside (through the ceiling or wall portal) and therefore there’s no chance of fumes getting pushed back through the ducting and into the non-source bathroom. Wonderful!

So as I consider buying this nifty device the question I keep having is whether or not I should pull the existing fans out of their respective bathrooms.

Q1: Will the fans that are at the ceiling impede the fan at the roof? If the roof-mounted fan is able to pull 120 cubic feet of air per minute, will it just be hampered by the bathroom fan that’s only able to move 80 cubic feet of air per minute? Will that burn out the motor or something?

Q2: Will the fan at the roof somehow damage a fan at the ceiling?
If the roof-mounted fan is able to draw 120 cubic feet of air per minute, will it force the bathroom fan (that’s normally only able to move 80 cubic feet of air per minute) to spin faster? Will that burn out bearings or something?