Any monoblock sub amp with stereo RCA inputs is going to sum them in parallel.. Also most music has its sub-bass frequencies mixed on each channel equally anyway...
Given if you have different bass tracks going on between Left & Right channels, you will not be able to discern a difference until the drivers are at least 1/4 wavelength separated from one another... At 340m/s at 100 hHz you would need a minimum distance of 0.858 meters before you could possibly discern between what Left and Right channels are playing...
It might be significant in a club, or outdoor venue... but not in your vehicle, and probably not significant in your house either...
I hope that Clarifies things for you.