You can't "move" a product to another shop. You have to recreate it and then asign the store you wish to see it in.
Go to the product page of the product you want to move. Use the "Edit Design" blue button to go into the design tool. Come back out (the Done button) and you will see "Sell It". There you can re-list the product in another shop.
Delete the old product, or if it has sales, leave it. If indeed you are keeping the older product you may want to alter the title or a tag of the newest product so you don't have "duplicates" in the MP.
