1 subwoofer vs 2 subwoofers

Here ya go.

http://www.doctorproaudio.com/doctor/calculadores_en.htm

In case you want to know the formula that was used to get the 136 Db estimate.  Generally speaking, anytime you double the cone area of your subs you will see a 3 Db increase.  Any time you double your power on the subs, this too will give you a 3 Db increase.  You have done both.  From 1 12 to 2 12s would give you 133 Db.  Then you doubled the power, theoretically you should be at 136.  Just in case you were asking yourself  "is there a rough formula for getting the db of the two 12" side by side if i know the db of the one"

Nice link, DYohn.  I'm adding that to my favorites folder pronto! 

Notice on that linked calculator, friskysquirrel, that there are boxes to choose:  "in phase" or "random phase".  You'll see that 2 identical woofers that apply to the categegory "in phase" (sometimes said to be correlated) will theoretically give the extra 3 db due to the additional cone area.  This cannot be said, though,if you are speaking of the front stereo soundstage or of subwoofers that are connected to the amp on separate channels.  The theory here would be that the two channels are separate signals and, although some of the content may be correlated, the assumption has to be that the signals vary in stereo and thus considered "random phase" (to apply the calculator's intent).

For your make believe scenario the subs would have to each receive exactly the same signal to calculate the additional 3 db for doubling cone area.  Bridging channels, using a mono amp, or combining signals prior to input to a two-channel amp are common ways to correlate the output.