when you feed the subwoofer out to the input of a powered sub, you ARE feeding exactly what the first sub is reproducing to the input of another amp.
After looking at that picture, it doesn't seem to be set up for daisy-chaining. You're probably best off by putting a couple of "Y splitters on the preamp out and feeding each leg to it's own sub..
FWIW, it's a good practice to make sure everything is powered down when kerfutzing with wires and cables.