That's a rather expensive external hard drive you have there Kozmo.
It can be done. You will need to partition the minis hard drive if you want to also use it as a computer. This means that the drive is split into 2 or more areas and would then appear in the Finder as if they were physically separate drives.
Consider that the mini probably came with an 80 or 120 GB hard drive. If you make two partitions, one should be at least 40 GB for the minis operating system and anything else you want to run on it. As your Book gets fuller, you might find yourself painted into a corner, when you have more on it than can be backed up.
Personally, I would get a real external FireWire drive, such as a fast LaCie with 1000 GB, partition that, have plenty of room to back up both computers and space for lots of other stuff too.