To be honest, it was a bit messy the first time, but their ears improved as we took turns and practiced listening. We found it helpful to separate the bass line players from the melodic players. Also, putting the groups into small circles so the students could see when people were playing improved the melodic interaction. Otherwise they students were simply playing every note on every beat. Once in a while that can be an interesting effect, but consistently done can make listening quite difficult.
I hope these visuals are helpful.