How to transfer music from a different IPOD to my music library?

I bought a new IPOD touch and have set that up on my new laptop. I have an old IPOD as well which is set up on my older computer. I want to consolidate BOTH libraries together as they both have very large libraries that took a long time to set up. I cannot find an answer on the IPOD help center. I am SURE I am not the first person to want to do this. Thanks so much!