I believe that the Pre needs to be put into Bi-Amp mode to activate both pairs of outputs. I would look into this as a solution. One pair of the outputs will feed the amps and one will feed the subs.
I have the 112V2s as a pair. I am running them in "stereo" from my pre, with a L output into one sub and the R output into the other. I have not slaved them with one sub feeding the signal to the other. The advantage of using my "stereo" setup is that each one can be tuned and DAROed independently and this helps reduce nulls. I believe that the slave mode is designed for applications of > two subwoofers where DSPing more than 2 subs dilutes the results because there is so much more sub coverage. Good luck and cheers.