He would be best to run one version of OS X on one disk drive and another on a second drive.
By that, I mean, Tiger is the optimum version of OS X to run FCP 5 on while Panther 10.3.7 or 8 is the best to run FCP HD on. Trying to run both would require 10.3.9 and Quicktime 7.01 which might cause conflicts with 4.5-HD.
Other that that would be a crap shoot.