Start by setting the phase to -180 degrees on both subs. This adjusts for the time delay introduced by the subwoofer’s amplifier.
Sub1 is aimed directly at the corner. This prevents the longer sound waves from bouncing around the room. Waves at the length of L1 are cancelled.
Sub2 is aimed directly at the side wall. This prevents the shorter sound waves from bouncing around the room. Waves at the length of L2 are cancelled. Sound waves a bit shorter or longer are partially cancelled. If you allow these bass waves to bounce around the room, they will contaminate your soundstage.
Sound waves that have been cancelled at length L1 are materialized at length L2, and vice-versa. This is the only arrangement where the front right, front left and both subs are all working together. You don’t need tons of power.
You need two subwoofers. It is physically impossible to materialize all the different length sound waves with one subwoofer. We have found that a lightweight sub (one with a light 10″ or 12″ driver) will preserve the sonic characteristics of your front left and right speakers better, so place that as Sub1. The sub that produces the very deep bass is better placed as Sub2. Front firing subwoofers are ideal in this configuration.