You can't really 'sync' per say, (that would create a lot more problems than it would solve, and it would require the 2 computers to talk to each other).
Anyway, you can use an app like Podworks for the iPhone to help you get music off the iPhone, and on to the other computer. Sort of manual syncing... http://www.scifihifi.com/podworks/
In other words no.
If you sync on one computer then you got all the songs listed on that playlist.
If you sync on another computer all the songs you got there are gone and replaced by the playlist on that new computer.
You can always copy all the songs you want from PC2 and paste them on PC1 and then sync.
I can do this, but only because I set up my iTunes library on an external HDD. I wanted a way to take my music with me while on vacation (I take my MBP as well), but retain the capability of syncing my iPod with any new podcasts I so desire. As both my iMac and my MBP point to the same iTunes library, there have been no problems syncing either of them with my iPod or iPhone.