Hey Niles:
If one has a dual filter set-up the filters should act in parallel. This would cut the resistance to flow in half( through the filters). Going in series would double the filter drag without producing any other advantage. If you are intent on reducing the drag produced by the filter(s) I would recommend
synthetic oil (mobil1, Red Line), lower viscosity
oil and a filter with more filter paper ( mobil1 filter 350 square inches of filter element vs Fram about 200 square inches).
Bob