Say you have one active subwoofer, and then add another. How many dB will you gain? 6db. Why is that? You gain 3dB by doubling the power (our system now have two identical amps instead of one), and also you now suddenly have two drivers, each moving the same amount of air as our single subwoofer. This means double the amount of air moved = 6dB gain. The result is similar to 4x power one a single subwoofer, assuming it could have coped with that.
So twice the power + twice the number of drivers.
This effect naturally works exactly the same with two active speakers.
I'm still not entirely convinced how you're getting 4x the sound power. isn't the additional energy provided by the additional amplifier is being used used to move the transducer?
Almost every source I have found online says if you have one speaker playing 100dB and another speaker playing at 100dB, their summation (assuming they're perfectly in phase) is 103dB, not 106. I don't understand how an active design changes any of that.