(Or, How to sync an iPhone with one computer and manually manage music and videos on another).
Note: This tutorial allows you to add music/videos/podcasts from multiple machines. If you just want to sync PIM (Contacts/Calenders etc) on one machine and media on another there’s a simpler way. On the computer you want to sync contacts/calendar with select only these items and perform a sync. Now, on the second machine enable Music/Photos/Etc and perform a second sync. You will be warned the media library will be deleted, but as there’s no media this is fine 
Because I do most of my music listening at work and my Calendar/Address book are kept in Outlook I have my iPhone setup to sync with my office PC. When I’m at home however I find I also want to put music/podcasts/videos on my iPhone for the weekend or trips.
Normally it’s only possible to sync media on an iPhone with one computer. If you try to use a second computer you get the warning below that "the iPhone XXX is synced with another iTunes library. Do you want to erase this iPhone and sync with this iTunes library"?
However, I’ve found an easy fix for this. In the instructions below I’ll show how to modify any iTunes Library so it can also be synced with your iPhone. You can then either manually manage your iPhone on a second computer, or sync different data on different machines (E.g. Music at home, Contacts/Calendar at work).
The instructions below use the example of modifying the iTunes library on my home Mac to work with my existing iPhone/PC setup. If your existing library is on a Mac, or both machines are of the same type, then the steps are identical.
