As far as I know, there is no way to migrate listings from one shop to another. Oh how I wish there were...and if someone knows of a way, please let us know!
I now have two shops (#1 shop - hand painted ornaments, #2 shop - artwork/digital coloring pages). I have several "best sellers" that I duplicate-list in BOTH shops. Here's the funny thing. As an example, in a shop #1 listing, I clearly identify and recommend going to shop #2 for the item (it's even a bit cheaper in shop #2). But I continue to sell it in BOTH shops. It's a nice problem to have (selling the same item in both shops), but at this rate I'm never going to be able to fully migrate the artwork to shop #2!
Bottom line, 2 shops is a pain! But if the items you sell are not cohesive, it's best to make the multiple-shop option work.