Not only are you benefitting below 20hz with the dual location placement, but you are also making your sub system more efficient. Reason? Because you are tackling other room modes by spreading them out, causing Audyssey to have to apply fewer, and less drastic filters to acheive a relatively smooth room response. This in turn will require less overall wattage from your amp if less boosting is needed in areas of the sub's passband. My guess is the differences in location would be more drastic if you measured the two locations WITHOUT running audyssey before hand.
Great point and the reason I asked bao01 to present his graphs prior to Audyssey RC (albeit that was in the REW thread). The other implied benefit here is less EQ from Audyssey when using better sub positioning.