My first grownup sub was the FV15HP. It is a beast! Same as you, open living room, cathedral ceiling, open to dining room. The single FV15HP was plenty to shake the whole house and then some. After about a year, and reading about dual subs the same as you, I got a second sub. This time a beast from SVS.
With 2 subs, set w/ Audyssey on my X4000, the house doesn't shake any harder, but a noticeable null at my wife's chair is gone. I could have likely found a placement for the first sub that would have helped the null. But as tolerant as my bride is, there are limitations on placement of that beast in the living room. (Ended up putting the 2nd one in the open dining room.)
I suspect one FV15HP will give you the full impact of a strong sub, even in your open space.
I suspect if you get 2 LV12s, you will always be curious about the impact of a full grown big boy sub. (You are a Jarhead, right?)
I suspect if you get 1 FV15HP, you will still be curious about the impact of having 2. (Still a Jarhead, right?)
So either way, you're gonna be curious and one day want to "upgrade". If you get the FV15HP now, upgrading is simply buying another. If you get 2 LV12s, upgrading may still mean getting 1 or 2 big boys, and having to dispose of the LVs, (limited space, right?) So I suggest the FV15HP now, then another whenever you're ready.
BTW, from your big brother out of the Canoe Club, we appreciate ya!