Lie to your iPhone and make it believe that it is synced/authorized on only one machine when in fact you are using it on two computers. Of course you always had the choice to sync contact, mails on one machine, songs, movies, applications, podcasts, etc on another machine. That was a long workaround as you have to manually select the content you want to sync. There is always this boring warning message “an iPhone can be synced with only one iTunes Library at a time. Erasing and syncing replaces the contents of this iPhone with the contents of this iTunes library.” However, if you manually select content, using contents from two libraries was possible, but that was somewhat manual.

Andrew Grant found a workaround and have made multiply syncing possible. What you need to do is copy the iTunes 16 digit/character ID from your iTunes XML file and edit and paste the  same ID in the iTunes Music Library XML and iTunes Library itl files on the other machine that you want to use for your iPhone. Curious to try this out? See the full instruction here.

Once the process is complete, your iPhone can no longer detect that it is being used on another machine as the identification ID is the same. This is how you can lie to your iPhone for your benefit. To perform the operation you need to be very careful, so says the discoverer of this trick.

Well and good for users. I have experimented this on two machines so far and will try on the third one soon. With one iTunes user account, I can authorize five machines. With this new trick, are there possibilities of manipulating the Apple Law Book that limits uthorization? The question here is whether Apple track downloads based on iTunes ID or IP of your machines. Not sure, will let you know the details very soon.