My first thought is like Quiltingcandy---make two twin size quilts that slip inside a king size duvet cover that has a seam sewn down the middle, the entire length of the cover, so each twin has its own "pocket" to slip into and hold it in place. This wouldn't look like a quilt on top as it would be smooth however. And it might be heavy with 4 layers of fabric when assembled so would use a lightweight batting. Most comforters made to go inside duvet covers are filled with down and are fluffy and light.
Seems like a long, separating zipper would work too. I am thinking I would attach it by using a long tab or double folded strip of the fabric used to make the back sewn into the back of the quilt binding down the side like a narrow hanging sleeve and attach the zipper tape to this "tab". The tab strip would be only an inch wide or slightly less so that when zipped up the outer binding edges of the two quilts would touch or slightly overlap.
Let us know if you give it a try and how it works out!!