I have an older home where the two bathrooms with showers did not have exhaust fans, only windows. I have added an exhaust fan for one bathroom and will add a fan for the other bathroom. The bathrooms are adjacent. I would route the duct for the second bathroom right next to the duct I already installed. The exhaust fans each have a flap valve to prevent air from the duct blowing down into the bathroom.

What are the opinions on connecting the second fan to the ductwork of the first fan to avoid putting another hole in the siding?