I do like having a quality sound system, but I'm not sure if I'm ready to drop $2000+ on just subs alone.
Then just get 1 sub now, (instead of subs). Get the second one when you're ready.
I also have a big room... w/ cathedral ceilings. Started w/ the Rythmik FV15HP. It filled the room w/ chest thumping bass. BUT due to the odd room shape, (like yours), the bass was not equal throughout the room. Depending on where I put the sub, some seats got a lot of thump, some got less.
I added a second sub. I set them up such that the bass volume stayed the same, but was more equal for each listening position.
When you use Audyssey, a meter, or any room correction method, there is a baseline volume for the sub(s) relative to your other speakers. (You can tweak to your liking, but you start w/ the baseline which is the relative volume intended by the recording sound engineers.) One FV15HP is completely adequate to meet that baseline, and more.
After a while, you will become more and more aware of how the bass volume changes as you move around the room. That's when you'll start saving for the second sub. And when you get it, while following the Audyssey or whatever setup instructions, you'll realize you're not setting the pair for more volume, but for more even coverage.
If you scrimp on the first sub, you'll never know what you're missing. So get one great sub now... a second one later.