I dont know about deeper bass, but a four 8's cab will shift more air and also have a volume incease over a two 8's cab, but of course, you lose the versatility of 2 cabs.
As for whether the Faital already has decent deep bass, i couldnt say. I can say that it depends on their design. I used to own two Markbass New York cabs that used four 6" drivers in each , and the low end on them was outrageous, so size is no guide. I guess the low end of those was due to having four of them, so maybe a bigger 2 x8 could achieve that.
The TSP's of the Faitals would tell you though, and if mounted in the correct volume cab ...