If you click on where it says "disABILTIES" at the top of this page it will take you to the Index. One of the top items is the "disABILTIES FAQ" which will gives lots of useful information, including where people rent ECVs. Post #4 there will be very useful for you.
Or, if you want an easier route, click on the link in my signature.
Two ECV will fit on a bus. When we went this past Dec. we were often told they are only allowing 2 ECV or wheelchairs on the Friendship boats at a time. This was at Epcot and the Boardwalk and Beach Club.