Re-purpose old water heater vent for bathroom exhaust fans
I would use the old gas vent with the existing exhaust fans without concern. The only issue I could see if one fan was on pressurizing the duct the second one might have trouble opening its back draft damper because of the pressure (both fans should have back draft dampers). I would check to verify that both will open when the other is running. If anything I would think a smaller fan would be needed to reduce the back pressure, but make sure it works and use it if it works.