Our bodies are only symmetrical to a first approximation. The contents of the chest and abdomen are obviously not symmetrical at all and our limbs are quite different when viewed in detail. Left-right differences in muscle size and strength, and speed, and dexterity of movement are significant and related to differences in representation in the brain. This specialization has to be adaptive at some level and, as RChung points out, there are energetic and anatomical reasons to question whether complete symmetry is optimal. This might be like the mythical "round" pedal stroke, which seems sort of intuitive on a naive engineering basis, but is biomechanical and energetic nonsense.
That said, dual PMs have got to be useful for working on real congenital asymmetries, bad habits, and injuries. I ride occasionally with a guy who might benefit. I can barely stand to watch what he does with his left ankle.