How do you add songs to an ipod without erasing the songs from a separate library?

we have ipod issues! My daughters all use different computers and different libraries and everything but they all have to use one computer (which is EXTREMELY slow!). They originally had their shared account on the slow computer where they download all of their songs. Then we got a new computer (but still use the old one) and when they tried to download songs from the new library on the new computer, they ended up erasing all of their old songs while downloading the new songs! what happened?? is there anyway we can download songs from two libraries?? HELP!! btw we have DELL computers, ipod nanos, and a slightly older version of itunes (the one that came before the most recent update) if any of this helps