You should have apologised to each buyer in turn, explaining what had gone wrong and then issued the buyer with a prepaid postage label for them to return the items to you at no cost to themselves. You would only have had to pay out for the labels if the buyers had actually used them, and if they had been able to see that there was a means of being able to return the items to you at no cost to themselves, whilst also being able to get the correct item that they'd paid for, then they may well have complied with your request.
Given that you've already told one of the buyers that he/she can keep the wrong item and you'll send the correct one you've effectively told the buyer "Forget about the fact that I've sent you something that somebody else has paid for - keep it and I'll just send you the correct item." You don't have the right to do this, as the item in question is no longer your property, and the rightful owner of that item could justifiably open an Item Not Received case in the eBay Resolution Centre and make things very awkward for you. Given that you have no way of being able to prove successful delivery of that item to the correct address you would have to refund the buyer in full before he/she had a chance to escalate the case to eBay and force the refund. If the buyer did have to escalate the case to eBay then eBay would automatically refund the buyer in full, in which case the buyer would get his/her money back but you'd end up with a defect on your selling account.
Going forwards, with regards to the buyer whose item you told the other buyer he/she could keep, send him/her an e-mail explaining your mistake and explaining that the person you erroneously sent it to will not return it. If you have an identical item then send that one to the buyer, explaining that you are providing an identical replacement item for no extra cost, and maybe issue a partial refund by way of apology. If, on the other hand, you do not have an identical item with which to replace the missing item then explain that to the buyer, apologise for the mix-up and state that you will issue a total refund for the item, then issue the refund straightaway.
At the end of the day you'll be very lucky if you can avoid a defect and/or negative feedback in relation to these transactions, but at the moment you really need to be focusing on damage limitation, so the sooner you contact these buyers and try to resolve the matter in a way that both buyers feel happy with the better.
