I think I saw this idea on someone's blog, but can't remember where. They punched several squares then layered them, matching the scallops, to create a larger square or rectangle. Then you place another panel over top so only the scallop edge shows. I think you do see the overlap, but in a pinch if you really wanted a larger mat, it would work.
***EDIT*** I just remembered which blog I saw this on. Beate's!
Here is a card where she made a scalloped mat with the scallop square punch.