How do you use two different itunes on one ipod

I share an iTunes with someone but when I got my iPod I didn't want all of my junk on there. I created another iTunes and now tried to use the iTunes I share on my iTunes but it keeps popping up with this message.

The iPod "ipod" is synced with another iTunes library. Do you want to erase this iPod and sync with this iTunes library?

An iPod can be synced with only one iTunes library at a time. Erasing and syncing replaces the contents of this iPod with the contents of this iTunes library.

Erase and Sync Cancel

I don't want to erase all of the songs on my iPod already but i still want some songs on my ipod that I don't have.

Please oh please help!