Regarding the Superdish, the answer is yes and no. With the current DishPro 4:3 switch, no. You will need two feeds to supply the dual tuners. In 2004, Dish will release a DishProPlus 4:4 switch that will allow you to split a single feed with a special diplexor to supply both inputs of a dual tuner receiver. I don'believe there is anyway to do this with a legacy Dish500 setup.
When DishPro Plus comes out next year you will be able to have this solution. This will work with DishPro lnbf's and DishPro receivers when it comes out next year. A DishPro Plus diplexer will be used at the receiver where the one wire from the dish will be diplexed into two wires.
This new technology will use band translating for single-cable two-tuner installations. This will not be a solution that will work as a single-cable to two single-tuner receivers. Both tuners will need to be in the same receiver for this to work.
Will it still be possible on the SuperDish for the DishPro Plus Twin lnbf to include an input for a third orbital location even though the 105 satellite will be used (having an lnbf with a different frequency) instead of 61.5/148 or will there have to use different switches for the lnbf running on a different frequency at 105?
They are supposed to be coming out with the DP+44 switch for four oribital slot locations out to four receivers and you can use a DishPro Plus diplexer as needed in each of those outputs for the dual tuner receivers. This would help if the 105/121 lnbf could be used with these switches seeing how there would be 4 orbital locations to receive if they come out with what some call a SuperDuperDish using 105 and 121. I have seen in a wiring diagram where a legacy receiver was connected to the DP+44switch without needing the DishPro adapter but also seen where there was a power inserter for this switch.