I was in the same situation a couple of months ago, I just emailed etihad guest, gave them both my account numbers and full names, including a description of what was my balance and latest transaction on both the accounts, and they merged them within a couple of days, so I was quite impressed. Especially considering the brilliant bank in question had spelled my name and email address wrong, it was surprisingly easy to get it fixed. Guess I was not the first who have had that problem...

Edit:
Etihad Guest wanted to cancel my old account and merge the accounts to my new account connected to the credit card as my miles form spending on the credit card would be credited to that account. But when I told them I already had canceled the creditcard they were happy to let me keep my old account and just transferred the creditcard miles to my old account.