Installers originally found this trick. A common satellite dish for SD receivers is the D500, and a common LNB/switch is the DPP twin. The DPP twin has only 2 DPP outputs, capable of feeding two duo receivers, for 4 tuners total, using separators. If the separators say sat1/sat2 rather than tv1/tv2, I apologize; I didn't go double check. Anyhow, it's possible to feed 3 Dish DP receivers with a single DPP twin LNB by feeding both separator sat1 outputs to the solo receivers, and both sat2 outputs to the duo receiver, hence my comment above. There are still only 4 tuners; that can't change. But the tuners could, if Dish would allow it, be in solo receivers, and not just duo receivers.
Note that DPP technology uses no additional frequencies beyond DishPro (bandstacked) technology. So the tuners in DP receivers can all see and select channels out of the DP signal. I claim it's a matter of a firmware upgrade to allow solo receivers to connect up to any side of a separator, and not just the sat1 output.