Both your buyers need to return the items to you, and it would be best to offer sincere apologies to both and ask them to open an item not as described case so they get the opportunity to return. In theory you should only refund once the items have safely landed back with you. Then you can re offer them to the buyers via BIN.
Now this may be difficult if a buyer has disposed of the item, had they contacted you to say they had received the wrong item? You could refund as a gesture of goodwill, but that is your choice.
