That's something I look into myself, as I'm planning to equip an ARE shell to the back. There are mattresses...
- that fit in between the wells...
- mattresses that go around them...
- and mattresses that go around and over them (AirBedz, as you noted).
After much research, I concluded that two people cannot sleep in between the wells alone, which was what I was originally planning so that I could build some over-the-well shelving. It will still work of course, but just for singular camping.
Everything I've read about the mattresses that go around the wells is that you tend to roll off into the gap between the protruding well and the mattresses well interior. That kind of discomfort is unacceptable. I'd rather buy a standard mattress that squeezes a little bit against the wells.
For the ones that go over wells, I'm pretty sure that's what you'll want. It's not only wide enough for two, but there'd be no rolling off. It wouldn't work in my case because then I wouldn't be able to put in the shelving. Also the ones that go over the wells sit rather high, requiring (IMO) a canopy lift like EZ Topper.
Though... camping in a short bed?