I think the assumption that adding a second sound source will help in accuracy is false. The issue with sound is it is a 4 dimensional thing. You have 3d axis of sound waves and their interference with each other then you also have to factor in time. The biggest issue facing achieving a good monitoring environment is ringing. This is most problematic in sub frequencies where the wavelengths are very long or longer than the room itself so can self oscillate making nodes. You are better off with a tight monitor if you can't fix your room issues. Adding a sub might cancel some nodes or smooth them but you will still be reflecting sub around the room.

You absolutely need to be absorbing or diffuse scattering those waves to deal with the real problems. I'm a bit annoyed anyone would suggest two subs, which is expensive compared to acoustic treatments is a solution.