Sounds like a simple question. You have 2 woofers with a sensitivity of 100db each and you put both of them in folded horn enclosure, so what is the actual rated sensitivity output then? How many DB does the folded horn enclosure add and how many more db sensitivity do you add using a second sub woofer?
+3dB for the second woofer and the rest would depend on the actual horn design (volume and length) is my understanding. Then there is the consideration of the impedance if you choose to series or parallel the woofers. That's just my guess though. I hope our speaker designer members can chime in. It's an interesting question,