That's what I said before, and even SVS recommended a second PB-10 to me as well when I was looking to upgrade. They said dual PB-10s would have a ton of output and should sound great. Unfortunately, I didn't have room for a second one. Buckeye is right on the money - it will be an upgrade, but it won't be a big enough one IMO, if you are looking for a noticeable increase in output. You'd have to step up to a NSD/2 or a Plus to get a significant jump.
Not just placement within the room, but the size and shape and the exact spot you sit all affect how you perceive the bass impact due to room modes. My old room had a pretty big dip higher up with the PB-10, and it was very noticeable. Nothing I did could fix that, so I ended up with my current sub. That sub had a very flat response in my old room, but in my new one, I have a bit of a drop off at the bottom likely because the room is larger and is no longer a suspended wood floor.
The PB-10 can reach the same low end as the PB-12, so two of them will get you much more output, but it won't get you much lower because they still have the same physical limits.
I'd send this one over to SVS as they will tell you without question what is up.