That's output, not extension.
Also, once you are below 16Hz, you aren't getting any intentional signal in 99.9999% of media. (for disclosure, I have an FV18 ran in 12Hz mode.)
If stacked, you will get closer to +6dB for doubling. If spread, it would completely depend on optimization; best case of between +3 to +6dB of doubling. So yes, it would take multiple FV18s to hit the same output as a Cap4000. But my point is how many people need or would even benefit from that? Especially outside of a dedicated theater room? Need I point out that a Cap4000 is almost the size of two FV18s and weighs 100lbs more?
I'm not saying there are no applications for such a product, of course there are. But when you are talking about people in a non-dedicated theater room mostly looking for music performance, it seems like pointless expense and dealing with a huge ugly box.
EDIT: actually, I'm getting the OP mixed up with another similar post, so I'll shut up with my OT comments. To the OP, two RS2's would perform awesomely if you need that much output and can live with the aesthetics (or can afford to pay Jeff for a custom finish).