With Legacy LNBs like old Dish and DirectTV you need 2 leads per satellite if you go to a multiple output switch. So that is not 4 outputs but 1 per satellite for each of 2 locations or a polarization for the switch. BTW DTV uses 101 and 119? while Dish uses 110 and 119 for its core and 119 has most of the basics but you will want 110 for some. The DTV dish may not do the skew needed for two Dish LNBs. An official dish shoulld be cheap. I think "Dual inputs" are outputs.
If you bought it new and have not had Dish before, the install should be low priced--see someone about Club Dish. You would get the dish, LNBs, Dish Pro DP21 or Legacy SW21 switches, and someone to point it, string cable, and get you going.
Try going to the point dish screen and with only one LNB select Dish 300. You may have to nudge the dish left or right, up or down. If that helps, then loosen the bolts and peak the dish. If you cannot get anything still, then you should reconsider your choice not to have it installed--I know, you like a challenge.
iluvmybrad: Yes, the 3-digit receivers will work with DP or Legacy.
-Ken