You can sync with two computers if both computers share the same iTunes library. I have my iTunes library on a remote portable drive attached to a Mac Mini. When I'm at home ,I boot up my MacBook Pro and Option launch iTunes. This asks the user where the iTunes library is located. You then point it to the location on remote drive. Now you can sync your iPhone. The iPhone sync is tied to the iTunes Library not the computer. When you are on the road, you can continue to add media etc. ITunes will add it to internal drive library. When you get back home , use the consolidate feature to transfer media from your portable notebook to the remote disk library. It works quite well , but a sync can be slow with a remote iTunes library. Try to Google for details.
Good luck
Mike