I've done this with one baby quilt. I wanted to use a minky backing but my piece of minky was just barely bigger than the top. I thread basted the top to the back and basted around the edge, then sewed 8" muslin strips all the way around it, then pinned it to my LA leaders as if it were just one piece of fabric. I also trimmed the miky before adding the muslin. It quilted just fine and when I was finished I removed the muslin strips and basting threads before I bound it.
Cari