I have a similar situation and have not tried it out. My buddy and I have been playing together at a nursing home. He is on acoustic guitar and singing and I'm on bass. Going fine, however, he wants to start using the organ -- long story -- he used to play and sing using the organ before it died. Well it's fixed now and I'm faced with your same situation.
I find if I'm having problems hearing the chord changes I rely upon the tonic pentatonic and use that.
Major = R-2-3-5-6 Natural minor = 1-b3-4, 5, b7
I think that may be the solution. As mentioned his left hand and my bass need to be in sync. But another thing to consider; playing pentatonics my riff and his melody should not compete with each other.
Interesting to see what the other guy/gals have to say.
http://www.last.fm/music/Norah+Jones/_/Cold+Cold+Heart