Originally Posted by Joe Reynolds

The combinator is what you need to do this. I know that this works because just for fun I just now did this. Maybe you got the wiring all screwed up. Let Reason do the wiring for right now. For now, just start over. Open a mixer and two nn-xt's. Load a sound into each one. Highlight each one while holding down the shift key. Choose "combine" from the edit menu. This does work.

Joe