When you run 1 sub per speaker aka stereo bass you don't get the primary benefit of multi-sub, which is giving a smoother frequency response due to better room mode distribution. Running the subs as mono below the high-pass point of your speakers is known as bass management.
Using REW you should take a spatial average of each speaker, with the stereo subs then again with the mono subs. Even moving the mic 6" in every direction you should find the stereo subs perform better. Moving mic method is good for this, much faster than merging individual measurements.
Yes, different sub placements are worthwhile, further apart usually being better. Front corners tend to provide the most overall output, mid-point of the walls usually has the smoothest frequency response.