There is a beautiful photo in Mothering Magazine's babywearing issue of a mother wearing her little ones, who even look around the same ages as mine, one in front and one in back, using one long piece of fabric. I have tried and tried to the same, and finally have succeeded. Really, it's too much weight, and the front child's isn't distributed as nicely as I would like, but Gibbie loved this carry. He requests it all the time!
The key is using a very long wrap and putting the back child on first. With the back wrap, like the one in my tutorial, completed, take the extra straps and make a criss-cross seat for the front child, tying them to lower straps instead of to eachother. Adjust as necessary!
Recommended for strength training and masochists.