How do I combine my duplicate Apple ID accounts?

How do I combine 2 accounts holding the same Apple ID given one of the accounts I cannot pull up on my computer? One is with an iPod Nano and the other with an iPod Mini. There are different tunes on each device. I would like to stop using the Nano and switch over to my Mini. I can use the Nano but I cannot access the Nano account to transfer my tunes.