DH and I have a joint account. If one of us buys something using the debit card can the other return it to the shop and get a credit to the card? Obviously we have a debit card each for the account but I'm not sure if they count as the same if a shop is crediting a card.
Depends why you are returning the goods. If it is simply that you want a refund because the goods are no longer what you want, then yes, it is likely the original card holder will need to return the goods.
However, if the goods are being returned because they are faulty in some way, then as long as you have the receipt, anyone can return the goods and get a refund. I had a row in a House of Fraser store over this when DW bought an item (using her debit card) that turned out to be faulty. I took the goods back and eventually, the manager had to be called to authorise the refund. The Sale of Goods Act, I believe, is your friend.