Don't try using two mono compressors because if, for example, you have an unusually loud drum sound panned hard left, only the left channel compressor will start working and the overall sound will drift to the right. A stereo compressor automatically compresses both channels, no matter where the peak sounds occur. Simply connect the left and right outputs from the mixer to the left and right inputs of the stereo compressor. The outputs from the compressor can then be connected to the mix down unit. An additional set of connections you'll see on some compressors are known as the side chain input and output jacks .
Some compressors refer to the side chain as a 'key input'.
Stereo compressors will have two sets of input and output jacks. One application might be to use the equalizer to remove all the low frequencies from the signal. When the treble-heavy signal is fed back into the side chain input, it tells the compressor to only respond to loud, trebly peaks in the signal. This is often used to remove sibilance from a vocal track. Another use for the side chain is for 'ducking', the old DJ trick of reducing the volume of the music when doing a voice-over.