they are either running them bridged together, meaning they are doubling there power by using them in sequence. For example say two amps puts out 1000 watts rms at 2 ohms each. well if you bridged the two amps together(few amps are capable of this) then you now have "technically" 1 amp at 2000 watts rms.
Or they are daisychaining them together with RCA wires(running a set of RCA's from one amp to another after running one set to the first amp from the head unit instead of both sets from the head unit because not all head units have multiple outlets.