You need to assign the sounds to the same Assign Group, particularly a mono group. Page 178 in the manual has a list of all the Assign Groups. Make sure that the layers in the Drum Preset that contain the hi hat sounds are both assigned to the same group. That way, when your open hi hat sounds, it will stop the closed hi hat from sounding... just like a real drum kit. The only problem with this is that any other sounds you use from the same layers will do the same thing. If you run into problems with snares stealing your hi hats, or any other issues, it may be best to use that hi hat Preset on its own Track.