So in theory the two subs have the highest potential MaxSPL. And in theory it could become 6 dB higher if you place them together and they do couple.
However, you would need to know a lot more about the methodology of the spec on the subs. It is unlikely that it is a dBa figure. It's much more likely to be quoted as dBc.
You also need more info from the promoter as to how to supply the number they are seeking. It has to be qualified at a "distance" (usually it's at 1m)
In the real world you are likely to end up with a number that is 15 dB (or more) below the quoted one and you probably need to shave that down by another 10-20 dB in the "c" to "a" conversion as the "a" spec won't pay much attention to the output of your sub. That number will depend on what frequency the manufacturer's spec is based on. If it's 200 Hz then the c-a filter will knock off about 10 dB and if it's 100 Hz the filter will knock off about 20 dB.
If they are asking for the potential output of your system then they probably want to know what the system can actually do. So maybe something like a pink noise source of some type and a Digital Sound Level Meter - RadioShack.com.
To make sense of this the promoter would have to specify max. dB(A) in what distance.
I would (seriously) just say 110dB(A) in 10ft but of course the actual level will be more likely at 100dB(A). Which can easily be 120 dB (SPL) depending on the music.