Kronos combination timbre volumes

[Flashman]

Hi folks... I'm fairly new to all this so forgive the potentially numpty question!

I'm trying to find a way of using my expression pedal to control the volume of selected timbres in a combination whilst not affecting others. Notably, timbres 1 and 2 are set with church organ programs and timbres 3 and up are all choir voices... I want to be able to fade the voices in and out with the foot pedal without affecting the organ volume... can't for the life of me work out how to do this!

Any suggestions or workarounds?