That and possibly handling.. It might well also reduce the size of the actuators to power them by having two smaller surfaces over one big one, I suppose in a combat situation you also have a certain amount of redundancy built in by doubling up on them.
On WW2 aircraft like the Lancaster it was to put the rudders in the propwash of the inboard engines, the accelerated air over the rudders and fins meant that you had rudder authority at lower speeds on take off etc and improved handling, as well as reducing the control surfaces size from a single item.