[Apple iPod] How to upload songs without erasing songs already there

Hi,
HELP!!
I have two separate Itunes accounts on my computer and have two
separate ipod music libraries there. I would like to download certain
songs from each of those separate libraries onto a third ipod. My
question is, how do I set itunes so that when it downloads the songs
it won't "update" everything and I'll lose the first round of songs
that I've already uploaded. Basically, I'd like to load songs onto
this third ipod and then load more songs from the second library
without itunes erasing the first round of songs.
Thanks, and sorry if this is confusing...
Jen

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