I have an Onkyo SR-876, and set up two additional subs for front left and right like you are thinking of doing (I don't buy that base between 40 & 120 Hz is not localizable). What I did was to run sub-cables from the receiver pre-amp front left and right out to the two subs. On my receiver, the pre-amp out is before the base management, so the cross-over to the sub-woofer (which I have set at 80 Hz) doesn't apply to the pre-amp outs. By hooking them up this way, I am able to tweak the fronts and main sub as normal, and then to dial in the front left and right subs using the gain and cross-over dials on the subs themselves.
I think this is a lot simpler than trying to run the full signal to the front left and right speakers through the speaker wire and have the subs cross over the speaker signal. If your receiver doesn't have pre-amp outs, you should try the tape-out. I'm pretty sure these outputs will bypass the base management also.
The only thing to be careful of is ground-loop isolation. If you get some strange ground-loop hum problems that are not solved by ground-loop isolators, see my other posting on what I did wrong and the extremely simple fix.