If you've already redeemed the card, you cannot transfer any part of the balance to another account. You'll need to regain access to the account though which you redeemed the card. Go to:
Click "Manage My Account", sign in, and go to the Password and Security section. Answer the security questions and you'll be taken to a screen where you can then change them. If you've forgotten your answers, there should be a link just under the security questions fields where you can have a reset email send to your Rescue email address. If the link for the email doesn't appear, as can happen if you didn't set a rescue email address or (apparently) have a .Mac email address, go here:
to report the issue to the iTunes Store.
Forgotten security questions - https://discussions.apple.com/message/18402551
More involved forgotten question issues - https://discussions.apple.com/thread/3961813
Frequently asked questions about Apple ID - http://support.apple.com/kb/HE37
If none of the above work, contact iTunes Support at http://www.apple.com/support/itunes/contact/ and follow the instructions to report the issue to the iTunes Store.
You want to avoid using multiple accounts. You cannot transfer items or credit from one account to another.