think about this, how the hell would you be able to control 2 disks? sure, freiza was able to do it in the show, but the disks are controlled by looking at the target, if you had 2, the disks would litterally be on top of eachother, and that would be useless. I think krillin should have a faster charge time on his disks, and maby a faster travel speed on all the disks, but thats all.