Motown experimented with two basses on some tracks...Some with Jamerson and Bob Babbitt, and a few with Jamerson and Tony Newton. Babbitt had stated that they really didn't come off well and sounded quite muddy...I suppose it could have something to do with Motowns recording equiptment and engineers just used to doing things a certain way, not conducive to certain types of recording innovation...or perhaps the parts were not written to emphasize the separation necessary for achieve good sonic quality???