If you have two equal (or nearly so) coolers, which have equal (or nearly so) air flow, you want to run them parallel. Heat transfer is a function of the temperature delta between the cooling air and the fluid. Having hotter fluid in both coolers means they both have the highest oil temps and thus the largest delta, and are the most efficient. See Carrol Smith on this and other race engineering subjects.
Doesn't Porsche do this with the water cooling radiators, one on each side?
Here is a picture of splitting the flow, and on a modified car it is not hard to do.
But on my stock class SC, with one Mazda cooler mounted in the center of the front valance under the bumper, I just ran them in series, with the better cooler (the Mazda) being the first in line, and the fender mounted Lemke getting the second shot. It is the less efficient cooler anyway, and some guys remove their fender mount cooler when they switch to a front mounted one. But it is easier in many ways just to leave it there. As long as you don't need to shed every ounce of weight it will help some.