Most people opt for two separate single mats. The bulk of a rolled up two person mat would make packing it up and fitting it on a bike difficult. I've found the Sea to Summit inflatable mats to be comfortable, quiet and reasonably dependable. They're not the lightest mats out there, and they're not the cheapest, but they're dependable enough to offset those two issues. The lightest mat in the world is just excess weight if it springs a leak on you part way through a trip. If you're truly determined to keep weight to a minimum, the folding closed cell foam pads like the ThermaRest Z-Rest are4 an option, but they're not the cushiest things to sleep on. They are cheaper than any inflatable pad I've seen, though, and they are completely and utterly leakproof.
Are you using sleeping bags or blankets?