What exactly is the point of doing so? Are you backing up the documents? Freeing up HD space?
I don't believe OS X is going to know to go to a 2nd HD to get to your documents everytime so you'll have to physically go to the drive. Although, you may be able to get away with putting an alias into the sidebar for easy access. I'm assuming you have a PowerMac with more than 1 HD bay? You can still do the same thing with an external HD (USB 2 and/or FW) but you'll just need to make sure the HD is turned on and mounted when you need access to your documents.