technically you could steer them all with your one mouse, but because all of your viewpoints on each character are slightly different, after a while they would be so far out of sync it would be a nightmare. not to mention client lag that causes your view of where you are to be different on each instance, in relation to other players in your group.

everyone on these forums use the /follow method. you will only find people using mouse sync for casting AOE spells simultaneously.