I know Disney does not have any. Also, in some of the magaines where I have seen
ECV ads I have never seen one shown.
A little over a year ago I saw a person in a Power Chair with a small (about one foot on a side) trailer which hooked onto his chair, and his wife stood on that and held onto the handles on the back of the chair. My wife and I discussed this with him, and he said it was definitely a custom accessory.
You might want to check with an off-site rental company. I have seen pictures of a two-passenger ECV, but it would require a special vehicle lift and would not fit on public tranportation and apparently could not be easily broken down to fit into a car's trunk.
An option might be to, if your mother needs it, do short-term wheelchair rental with pushing. My wife and I try to get a midday break daily, adn also I keep the ECV speed down to her pace (unless she is sending me on an errand).