Everyone's probably already figured this out, but it just came to me the other day. I needed two spools of thread for use in a twin needle top stitch application. But Joanne's only had 1 spool of the color I needed. Since I was using wooly nylon in the bobbin I used the regular thread bobbin in my secondary spool holder. The bobbin fit fine and the twin needle stitching came out great. I can't believe I've been buying two spools of thread all this time.