IOW, 2 x 200W subs co-located = 6dB = 1 x 800W sub.
Maybe I misunderstood craig john. Perhaps he'll chime in to clarify.
Another way to look at it: For every doubling of power, you get a 3db increase. I believe the assumption is the driver size stays the same (so you don't get any displacement gains); therefore 200 x 2 = 400w (3db), 400w x 2 = 800w (6db).
It would take the same sub powered by 800w (6db gain) to equal the output of 2 of those sames subs collocated at 200w each (6db gain). This is of course assuming the driver can handle the extra power.
So, you are correct. Two co-located subs is double the output (not quadruple). To equal the output of the two colocated subs for the same single sub, you'll need quadruple the power. This is the way I understand it, anyhow...
I think you and I just said the same thing, didn't we?
Where's the misunderstanding?