Steps to Sync Your iPhone With Multiple Computers (Mac)

This tutorial is devided into two parts, the first part will instruct you on how to obtain the iTunes Library ID from your main computer. The second part will then modify your secondary computer to use the same iTunes Library ID, thus allowing for sync on both.

Step One
Open a new Finder window by clicking the Finder icon in the dock.

Step Two
Select Go to Folder... from the Go menu.

Step Three
Input ~/Music/iTunes into the textfield then click the Go button.

Step Four
Double click the file named iTunes Music Library.xml to open it in TextEdit.

Step Five
Write down the value of the Library Persistent ID key. Take care not to modify the file before closing. It should look something like D501EB4887717F8F.

Step Six
Make note of the current Library Persistent ID then change the Library Persistent ID key to the one we wrote down from Part One and save the file.

Step Seven
Now we will need to open iTunes Library using a HexEditor. You can download HexEdit from here

Step Eight
Once you have opened iTunes Library use HexEdit to search for the old Library Persistent ID we replaced in Step Five. Do this by clicking Find from the Menubar. Make sure to select Hex as your search type.

Step Eight
Once found replace the old Library Persistent ID with the one from Part One and save the file.

Step Nine
You can now launch iTunes and connect your iPhone to the new computer for syncing. Select your iPhone from the list of devices on the left. Notice if now select Manually manage music and videos from the Summary Tab you will no longer be prompted to erase your data, nor will the files on your iPhone be grayed out.

These are the steps that enable you to sync your iPhone with multiple computers on Mac OS X, we cannot sure that it is the easiest way, but it is efficient anyway.