I only have 1 sub per theatre.
But every one of my theatres(sans bedroom) runs full range speakers crossed over at 40hz. Ergo, I only need 1 sub per theater.
It helps that I listen to SACD/DVD-A and prefer to listen to those on "direct" mode(bypasses bass management).
I get what you are trying to do. And if my rear channel speakers(and yes...these are the rear channels) weren't the DCM TF350, BIC V820 and Infinity Crescendo 3007(three theatres)...I would use a subwoofer on the rear channel.
Your main issue is, your AVR only allows a 100hz crossover(I assume if you put your front small, so does your rear channel have to be set small). What you are trying to accomplish won't work with a 100hz crossover.
A newer AVR(if you care to use subwoofers, instead of larger speakers) will allow you to set the front and rear at 40hz. Then...
An AVR with pre-outs...
1. You can then run the fronts and/or rears at 40hz to a subwoofer via high level, which would let the subwoofer play 40-80hz...the "sats" play 80hz and up.
2. Run the "sats" via the speaker terminals and run the pre-outs to the subwoofer. Then your "sats" are running 40hz and up and you get additional bass fill from the subwoofer from 40hz to "whatever you set it".