My husband's family is planning a trip with us next year to WDW. His mom will be using an ECV, and his grandma will be using an ECV. Will they have any trouble with using Disney transportation or will they both be able to board the same bus? Or is it just one per bus?
depends on what type of bus. Some take 2, others take 3. but I think 2 is the minimum. If the bus arrives and there is already an ECV on it, they'd have to wait if they want to go together.
All buses can take two ecvs some can take three and a few can take four ecvs. If the bus show up and has another guest and you don't want to split up you have to wait and the driver will call for another bus.
When there were two of us with ECV's in our party, we rarely had to wait for another bus to avoid being split up, but we were there at a not-so-busy time and may have just gotten really lucky. (When there were other people with ECV's ahead of us, we had to wait for the next one, of course.)