I get what you're saying here but I can't see how this would drive efficiency. The 42 has ~120 daily trips, compared to the 61's ~50.
If you were to extended the 42 north full-time, almost all runs would be nearly empty north of New Rochelle.
If you were to extended the 42 north part-time, creating some sort of 42P variant, you'd most likely net the same amount of runs as it currently stands.