The chance of each version of Stun being applied is calculated separately, so that part does stack. I don't know if the effect stacks if both versions are triggered, or if just the larger is applied.
A word of warning; mixing any two of Frost on a weapon, Stun on a weapon and the passive Stun skill can sometimes result in creatures hanging in mid death animation, and not dropping items or giving experience. An area effect spell that doesn't need a target, such as Hell Spikes, can force the animation to end, though. Some people also reported that waiting a bit or sleeping would also work.
