Bathroom Venting Question

I have a 50 year old raised ranch. We have two bathrooms on the raised level right next to each other so that one has a window and one does not. It actually has the appearance that it was at one time one larger bathroom but, somebody added a bathroom to the master bedroom. The bathroom closest to the back of the house (with the window) was never vented at all. The hall bath was vented straight into the attic. Luckily, it doesn’t seem as though any damage was done. We have a very low roof and it would be nearly impossible to vent this to the soffit as I can not fit there and I am not very large. I am thinking of venting the two bathrooms to the gable side. It will be about a 15 foot run to the gable. My question is if I pitch the ductwork toward the gable, can I vent the two out of one vent. I'd rather not cut two holes into the side of my house. Much less have two vents within 4 feet of each other from the exterior view. I feel like if it is pitched on a slope over 15 feet, it would be unlikely for the moisture to work its way back to the other bathroom.
Any help would be appreciated!
Thanks!

It all depends what the code allows at your location. In many locations the vent must extend though and above the roof.

Surely you can combine two vents but what is this about ductwork¿

What I'm hoping to be able to accomplish is to have the ductwork run from both bathrooms seperately, and meet at a y or a T attachment to enter the 4 inch duct at the gable.

I think the subject is about ducts for ventilation not a DWV vent........

You'll have to check local codes on what's allowed, but as a practical matter, you can't join two 4 inch lines into one 4 inch line without a severe drop in performance if they are ever used at the same time. Any standard size less than 6 inches (round, or rectangular equivalent) will be a reduction when two 4's are fed into it.