We used to use a double air mattress, but found that when one person rolls over without ridiculous amounts of care, the other one gets ‘trampolined’ a bit – neither of us are fatties, it’s just the poor design of the mattresses. So we got two singles and put them side by side with a double fitted sheet over them – much improved. But I still find them to be cold – even if my upper side is toasty under the duvet (proper namby-pamby glampers, us!) the side against the mattress was always cold.
So now the mrs has her single air mattress, while I use her Exped down-filled 3/4 length mattress over a foam mat. Toasty as. So I guess I’d recommend a couple of Expeds rather than a double airbed, although they aren’t cheap. Budget option would be to get about three cheap foam mats stacked on top of each other. Warmth and comfort, and if you’re camping out of the car the space shouldn’t be a problem (fold them in half and pack them flat on the boot floor for best packing/space efficiency.