Each module can only fire one at a time (i think?) so firing multiple "at once" would require two modules. However, using two at the exact same time would be...well, hot.
I have my SCBs and Heat Sinks bound, so I don't need to have them in fire groups. My process is that I fire one, and watch the heat race up and when it gets to around 80% I fire another. This way I use two per heat sink, and if I wait five seconds after firing the first, then the second SCB will come from the same module as the first, which will be the largest.
Also, consider going with the "B" class SCBs. These have fewer MJ than the A class, but the B Class has one additional SCB than the A class, making the total MJ that the module can restore higher for the B class than the A class.