If you are set up correctly, surround information at a lower frequency than the crossover point will automatically be added to the LFE channel and come out the subwoofer. This is a non issue since low frequency bass is non-directional and doesn't need to originate from behind you.
However, there really is something to adding additional subwoofers to your system. Where the subs are physically placed in the room will be a function of the room itself, the listening environment. But the additional sub would be wasted if you were only going to feed it from the surround channel's frequencies. The surrounds simply don't have that much bass info to reproduce in normal circumstances and it is reproduced by the subwoofer anyway. If you wanted to fill in some surround bass, simply place the second subwoofer behind or beside you (integrated properly, of course) fed from the main LFE channel.
