I don’t think it’s possible, but off the top of my head you would need two HDD’s both with OS X installed on them. Then you would need to install FCP 6 on one, remove it, boot the other drive and install FCP 7 on it. Then re-insert the FCP 6 drive, boot from it and see if you can run FCP 7 from the other drive.
This is all purely hypothetical, I’m pretty sure you would run into some permission errors, but I’m just spitballing.
Tom