2LV12Rs verse 1 FV15HP was just well discussed in the Rythmik thread, check it out.
In the end Brian said 1 FV15HP. If you co-locate the LV12Rs you get the same output but lose the nicer PEQ features of the FV15HP. Of course you can get a smoother natural response with two LV12Rs if you spread them out, but then they have less output. 2 co-located identical adds 6db. Spread out it varies but can be as little as 3db. A FV15HP has a 6db advantage over a LV12R. Also with duals if you don't have measurement gear you may not get them integrated well.