All the music & videos on the iTouch should also be stored within iTunes on the computer so adding a second iTouch will be a simple matter of plugging it in, give it a unique name when formatting/registering and let it copy all the stuff from iTunes back to the iTouch.
The only things that won't automatically copy will be information created on the old iTouch such as Contacts, Calendar entries. However if you have all these synching with iCal, AddressBook etc on the Mac you can just make sure to run a full sync and backup of the old iTouch before attaching the new one.
You can explore and extract information from the old iTouch backup using this program
http://supercrazyawesome.com/
