Thanks John, makes a lot of sense. In a way I would prefer to use the dual outlets for the front and the single outlet for the rear, however I am concerned about line pressures.
I just calculated it and the difference in piston area (output pressure) is 10% between a 19mm bore and a 20mm bore.
20mm bore = 314.16 Sq. mm.
19mm bore = 283.63 Sq. mm.
My guess is that BMW will have used a very small rear piston at the back, which are also probably disks. I have about 17mm piston (not actually measured it) drum brakes. So to keep the pressure differential safe I feel I should be keeping the bias towards higher pressure at the front, else I may have trouble with back brakes locking up in the wet even under light braking. I once had a Mini with a seized pressure limiter at the back, that was a regular for swapping ends unexpectedly!
While with a Mini that's kinda acceptable, with a Marlin I think I may loose friends and passengers pretty quick. LOL