multiple vents --H20 htr plus gas dryer
 

My small, vintage 1923, 900 sq foot house in California has a water heater in the "basement" (small area under house probably used to store vegetables at one time, 6' concrete floor to bottom of house floor joists), and a gas dryer above in the main house. I'd like to combine vents, but not sure if i can use double wall with a "Y" connecting the two gas appliances. Any thoughts or best practices?

No you can't combine a flue with an exhaust vent.

Gas dryers by code. Must be on their own vent.