Venting two bathroom exhaust fans through one roof duct won’t work, sorry to say. But you can install one fan in your attic for both bathrooms, and make your bathroom quieter, too.
One in-line centrifugal fan can be mounted in the attic to exhaust the moisture from two bathrooms.
Both bathrooms are vented by a single in-line fan that has one exhaust vent running through the roof.
Each bathroom has its own exhaust fan; each fan vents separately out the roof.
If you have two bathrooms that are close together and one has an exhaust fan and the other doesn't, you might be wondering if you can tie a new exhaust duct into the existing one. Well, you can't! You'd often blow air from one bathroom into the other, and local building inspectors wouldn't approve it.
But while you can't have two fans with one vent, you can make one fan and one vent serve two bathrooms. This setup requires an in-line centrifugal fan mounted in the attic drawing air simultaneously from both bathrooms (see photo). A grille in each bathroom attaches to ducts, which then fasten to a “Y” connector at the fan. A single exhaust exits through the roof (Figure A). You mount a switch in each bathroom. This system is quiet, too. Because the fan is in the attic, you'll hardly hear it. Look for the special fans (starting at $160) at heating-cooling equipment dealers. We used a system by Continental Fan Manufacturing (continentalfan.com) Fantech makes a similar fan (fantech).
It's less expensive to simply add a fan in the second bath and vent it separately (Figure B), although that will entail cutting another hole in the siding or roof. If you go this route, consider a fan with a built-in humidity sensor. It detects a rapid increase in humidity, like during a shower, and automatically turns on the fan. The sensor switches off the fan when the humidity drops. The fans with this upgrade aren't cheap, but they eliminate the need for a timer switch. Two companies that make the fans are Broan (broan.com) and Panasonic (panasonic.com).