I'm mainly going to comment to defend the quilt as a concept and Enlightened in particular. I don't have much to say about using a double quilt, or sleeping bag, and since my only experience comes from home, where I have given up trying to share a blanket with my partner, I think it would end badly.

If you two are compatible sleepers, with similar temperature ranges, and fewer tendencies to steal the blankets, then a two person quilt might work wonders. I have a 1 person quilt that I love, it's just as warm as a sleeping bag (for most weather, when it starts to get far below freezing, I switch back to a bag), and it's a lot more comfortable. There's nothing like the freedom of movement, flexibility, and ability to regulate temperature that comes with a quilt. Plus, they tend to be a lot lighter.

It's not quite a fair comparison, the Enlightened quilt to the Big Agnes bag; for the same temperature range, the quilt is half the weight, and probably considerably less bulk. It's up to you if this matters to you, but I'd be pretty happy to not have those extra two pounds to carry.