two 12" L7's will be louder (SPL)...how much louder!? Not sure.
SQ? no not at all...the L7's are not very good SQ (sound quality) wise.
As for Decks...it's all personal preference, and what kind of accessories you need/require. MP3, iPOD, USB, Auxiliary inputs for external devices...etc....how many RCA outputs for (front, rear, subwoofer) if you require those...etc...
I personally like Kenwood Excelon series decks.
PS. I've owned a Fi BTL. And my friend has L7's....I prefer Fi personally....But two L7's should beat a single Fi.
But if you are still "shopping"....might I suggest (if you are looking for SQ), is the Fi Q. Another one of my friends had two Fi Q's...and when weighed against my Fi BTL, killed my BTL in SPL and SQ....And is cheaper. I believe the price of two Q's....is just a tiny bit more money than 1 Fi BTL.
Haven't checked the prices in a while.