The ability to merge accounts is something Apple has mentioned as a goal, but hey've set no timeline, nor have they even hinted at what kind of limitations or proofs would be needed.
So, don't hold your breath.
What you could do is log in to one account and use the iTunes Gift option to gift apps to yourself on the other account. You'll probably end up leaving a few pennies behind, but you can get the bulk of your Gift cards value into a single account that way. It will just take a little while (unless you go on a buying spree).