Good man! Should you decide to build a second cab, here's how I see it, having used every combination.
15/6+12/6cube- Will sound more or less the same as the single 15/6, but 'more.' Louder and clearer...super versatile setup, as I agree that you'll be very surprised to find how many situations the 12/6 handles very well on its own.
15/6+15/6-You'll get a more muscular effect than the 15/6+12/6 combo, which may have something to do with having that second 3015LF up by your head

My favorite rock and roll excess stack is two FEARLESS F115's stacked, which is a similar idea.
15/6+15sub- Monstrous sounding and seemingly endless SPL handling...put some serious power to this rig and you'll be afraid the ceiling is gonna fall down at some point

Some find this stack to be perfection, while some find the added sub can obscure mid clarity if one isn't careful with eq.