Accidentally redeemed eon bundle on wrong account.

I would suggest to double check to make sure you are logged into the correct account first before any redeem.

If this cannot be fixed on the Xbox end (which I don't believe it can), I advise you to contact EPIC Games directly and explain the situation and I'm sure as long as you provide proof that you own both accounts etc that they will figure out what to do
from there. https://www.epicgames.com/site/en-US/customer-service

Isn't that pointing out the obvious? That doesn't help him at all

Unfortunately you can't transfer content among accounts. As for a refund you can always try with the refund form: https://support.xbox.com/en-US/billing/xbox-subscriptions/digital-goods-subscriptions-refund

Indeed you cannot transfer content between accounts, but you can change which account your epic games account is connected to.