A separator won't work to feed both tuners without a DishPro Plus switch at the head end. I see a DPP44 in your future. Or maybe the cheaper DPP33 if you can find one for the 2 dual-tuner receivers. What switches are you running now? DP34's? I ran happily for years with a DP34 cascaded to a DPP33.
The house's with the VIP612's have DPP44's
Rob