Not sure what you want to hunt, but the only way you are going to get low pressure loads for .375 Flanged Magnum that shoot parallel in a double, is to shoot lighter pills.

Here is a load that shot quite well for me;

Projectile: 235 gr. Speer Semi Spitzer
Case: BELL
Primer:Remington 91/2
Powder: IMR 4064 - 72 gr.
MV: 2650 fps

For "plinking" and use on light-skinned game, the Hornady (or Taipan) 220 gr. Flat Nose pills with 1-2 extra grains of powder can be driven at MV's up around 2750 fps.