Over the weekend I watched the movie Love and Mercy. In one scene Brian Wilson and Carol Kaye are talking about the bass track on a song. She asks him about a part where she is going to play one bass line on the electric and another person is going to play a different bass line on the upright. The way she phrased the question made me think the two parts were going to be a harmony of some sort. Is this correct? Can someone explain this? Was this common practice back in the day?