No wonder you're confused. Single, Dual and Twin are 3 different things - click "here's why" in my sig. You either have 2 duals (maybe 2 singles) or 1 Twin. If you have an SW21 and only a Dish500 (no extra dish for a 3rd sat), then you have 2 separate LNBs. Those are usually duals but could be singles. The cheapest solution is to get another SW21 and connect it just like the first one. Doing that, or replacing the SW21 with an SW44 or SW64, requires dual output LNBs.
You were also looking for a way to "split" a single RG6 line to the 2 inputs of the 942. That is possible, but not quite as you proposed. Your legacy LNBs and switch will have to be replaced with DishPro Plus. Since you will only have the 942 and possibly also the 811, you need the DPP Twin. The DPP Twin has 2 DP LNBs and a DP Plus switch in a single assembly that would replace your LNBs and SW21. At the back of the receiver you would use the DP Separator that comes with the 942. You have the option of adding another dish with a DP Single or DP Dual. Costs more than an SW21 but saves you the hassle of running another line to the 942.
Either way you'll be spending some time on a ladder and removing LNBs whether it's to replace them or get at the second outputs.
The DP Twin is fine but requires you to run a second line of RG6 to the 942. If you wanted to reinstate the 811 someday you would have to replace it with a DPP Twin or add a DP34 or DPP44 switch.
DishPro and DP Plus are explained at
http://ekb.dbstalk.com/217.