Depends on the band style, do you need lots of low end of a 4x12, if the bass player is doing his job?
A lot of the biggest pro metal tours aren't using the 4x12s on stage for the front of house mix -4x12s are fake or just for stage reference for the band. and they have a twin or single mic'd or even a profiler tucked away on the side stage to feed the FOH mix (what that audience hear)
Why? because often the very bottom end of a 4x12 isnt as usuable in a great mix, or doesnt sound good and makes the mix too muddy.
SO I wouldnt worry about a 2x12 on low end unless you have decided what you need in the context of a band.