Hm, Werticus I think you may have got something mixed up somewhere -- "you must not use a driver that is smaller in size than the wavelength its trying to produce" would mean that you would need a 20-foot driver to produce 50Hz...
But, to the question, there are two issues, the first is volume displacement which as Josh says is almost certain to be significantly larger for a single 12" than for two 6.5" drivers. Apart from the 3-4 times larger surface area of a 12" driver it is likely to have a higher Xmax as well. Volume = area (Sd) x Xmax.
Now, I personally happen to think that larger drivers sound more natural than smaller ones. I attribute this to the lower excursion needed to produce a given SPL, and surmise this to be due to reduced intermodulation distortion (because the drivers moves a smaller distance) and more headroom. I have no proof, but so far it's proved to be a good working theory -- small (and I consider 6.5" small... no jokes please) drivers sound like small drivers.
Sooner or later someone will do you the favor of working out that for a given SPL you need only a certain amount of power/Sd/Vd/whatever. Almost always they forget that you need a minimum 20 dB of dynamic headroom, and preferably more, much more. To put it another way, smaller drivers will sound "strained" much more easily than larger ones, because they are compressing the dynamic peaks. It doesn't mean they sound bad, I listen to 5" drivers regularly but the dynamic limitations are obvious.
HTH
