I don't do the calculations, but I think two 8" is greater driver area than one 12". Should get greater gain.
Effective cone or piston diameter is usually smaller than the nominal frame size of 8" or 12", but based on those numbers, 2 x 8" = ~ 100 in² and 1 x 12" = ~ 113 in².
So it's close, with a possible slightly larger cone area for the single 12", but as Peter points out, the dual 8" can probably give a smoother overall room response.
Using the same amplifier in each, the total power is double for the two 8" vs the single 12". Interestingly, the frequency response is the same for both (28Hz - 160Hz). So if you have the extra room and placement options and can afford the additional 20% in cost, the dual 8" subs seem like the way to go.
Steve