My opinion, a disk is hardware and eventually it can wear out or break. The more times you run defrag, the more likeely you will eventually wear it our (personal opinion here, based upon little or no research on the topic). Why would you need to do it every few days? What problems are you having that would cause you to do this.
I'd suggest once a month.
As to both at the same time, (again based upon personal opinion) what would happen if both are trying to move a file at the same time, you could end up in the Delat Quadrant or even go back in time and meet your mother five minutes before she met your father for the first time. (Pardon my attempts at humor, but...) you are delving into an area where something bad can happen for no real benefit.
I wouldn't over tax the disk and run it periodically, unless you are having real problems.
Hello all, How often should you run disc. Defrag? Also is it benificial to run two defrag's? I run XP and Nortons at the same time every few days. Any comments? Thanks,D.