Call the credit-card company's customer service department. The phone number is located on your monthly statement or the back of your credit card. Tell the representative you wish to transfer a portion of one credit card's account limit to another card. These credit cards may need to have been issued by the same company in order for this process to work.
Let the representative know which account should have a portion of its limit transferred to another. Give her the last four digits of the card that will have its limit reduced. That portion of your credit limit will be added to your other credit card.
Make sure the process has been completed to your satisfaction. Verify with the representative the new credit limit for both credit cards. You will still have the same amount of credit when you add the credit limits together. For example if both credit cards had limits of $5,000 before your transfer, one may now have a limit of $6,000 and the other $4,000, but they still equal $10,000.