Just to clarify, we had issues with the headroom, which is a function of width, not length. The sides on a ridgeline (A-frame) design are sloped and, for two people to be comfortable side-by-side, you need a surprising amount of extra width to allow for the unusable space along the sides. And if you're using air mattresses (e.g. Neoairs) which are thicker, they will put you that much closer to the sides.
You might try stringing up a bedsheet between two chairs at the height and width of your design, slide your pads under it side by side and make sure you can both fit under it without feeling claustraphobic. If one of you has wide shoulders and is a side-sleeper on an air-mattress, you may have problems...