Though this isn't a very common setup, from time-to-time customers want to use two S8s in their system and it isn't really hard to setup.
Each of our powered speakers, with the exception of the B2, and the N22 have an RCA output that you can use to connect a powered subwoofer, such as the S8. In a setup where you're running two S8s, you simply connect each one to either channel of the RCA output; this basically means that you would use an RCA cable to connect one S8 to the left (red) channel and another RCA cable to connect the other S8 to the right (white) channel. This will send audio out to both S8s at the same time. After that, all you'd need to do is adjust the crossover and phase to your taste and enjoy!