I think taking a sleep number for car camping would rock
Yes, sharing a quilt can save a lot of weight, you only need about 25 % more material, and you get a higher warmth rating as well, since two bodies are creating heat, theoretically, insulation can be cut in half, but in reality, it's probably more like a 40 % reduction. So for instance, the same amount of insulation that a single person requires for 30 degrees, should get two people to 15 - 30 degrees below zero, assuming the same amount of space.
As for a pad, take two inflatables and attach them with a webbing in 2 or 3 spots.
