Your question is unclear and confusing.
If you mean you used to Sync to an Old computer and you now have a New computer you wish to Sync to...
To have EVERYTHING as it is Now...
From your OLD computer...
Copy your ENTIRE iTunes FOLDER to an External Drive... and then from the External Drive to your New Computer..
Full Details Here > http://support.apple.com/kb/HT1751
An Added Bonus is that you will have a Backup of iTunes.
If you mean you wish to Sync to 2 Different iTunes Library...
You can Only Sync to One iTunes Library at a time.
I have all of my songs on the older computer, but it will not let me sync songs on to it because iTunes is synced on another computer. The new computer has 25 songs, and wouldn't let me Backup my files on to my computer. It also wouldn't let me sync. I accidentally applied the new settings to my iPod, so it only has 25 songs, versus my 155 total songs. I don't know how to unsync a computer. I shared my old library on my local network, and I want to know how I can access the songs.