I was looking in to how to do this my self and i came across something called "MiniSpy SATA Hard Drive Adapter" this is what it says about it
QUOTE
Producer: Talismoon
However you may look at things, the fact is that 20GB of storage space for your average 360 gamer is just not enough. Talismoon have long kept their prying eyes on the situation at hand and have developed two innovative solutions to the problem described. Both the Minispy and its bigger brother, the Minispy USB, allow you to free up space on a 360 hard drive via acting as a bridge between the drive and a PC.
In conjunction with powerful free file management software currently available such as Xplorer 360, the Minispy will allow you to transfer game saves, mp3s, demos and other media as required. The possibility of all your drive information being backed up safely on an everyday home PC is now a reality. This not only gives you extra security for your files, but also allows you to squeeze out every amount of 360 hard drive storage space available as and when you need it! The Minispy is so simple to use, just pop your hard drive off your 360 and plug it into the Minispy, you will now be ready to begin your journey on the quest for mass storage capacity.
Maybe you could tranfair the info onto your pc harddrive then to the other 360 HD