It seems to be a random 50/50 chance of either. It's strange that when you have the feat you have a radial option to use the specific one, but I couldn't find a way to tell it by script which to use. Even more annoyingly, they have seperate actions in the 2da:
29 CalledShotLeg 518 ir_calledleg
30 CalledShotArm 517 ir_calledarm
Maybe someone has managed to hook them individually for NWNX, it should be possible to do, but I never noticed the option for it in any of the features updates.
