The best waay would be to set up a MobileMe account to keep your Macs and iPhone in sync. However, if you don't want to pay for MobileMe you can use your iPhone a syncing device.
First, you must set your iTunes on both Macs to the same iTunes Library ID. See
http://www.moviesmac.com/tutorial/sync-iphone-with-multiple-macs.html for details.
Now, each time when you sync to the second computer, you will get a message like this:
+The information on the iPhone is synced with another user account. Do you want to sync this iPhone with the information from this user account instead?+
+“Merge Info” merges the information on this iPhone with the information from this user account.+
+“Replace Info” replaces the information on this iPhone with the information from this user account.+
Choose "Merge Info" here. But be careful: it's a merge, not a sync. This works fine for new events in your calender or if you have changed some details. If you have deleted or moved an event on one system and the old event is still present on the other system, it will be on both devices after the merge. Conflicts might also appear if you change the main title of an event.
So MobileMe is the better way 🙂