If I understand your proposed setup correctly, you're planning to bring two more cables in from your dish, i.e. you have two cables coming in now from a single "dual" LNB and you want to add two more cables in.
Whether you can just add a second single "dual" LNB to your current dish depends on whether you want additional programming (esp. HD) or not. All HD and some SD programming is on the 82 satellite. In order to pick up the 82 satellite, you need an "elliptical" 20" dish and it needs to be skewed slightly. This allows the second LNB to receive the signal from 82. If you don't care about HD/additional programming, you can just add a second single "dual" LNB to your existing dish. This will give you four cables all carrying signals from the 91 satellite.
I'm not sure your plan to use separators to double the number of potential receiver hookups from four to eight will work. I don't know anything about the types of separators people use to split a single cable to feed a two-tuner 9200 receiver, but I'm not sure they'll work with legacy LNBs. If they do work, and if you don't care about HD/extra programming, I don't think you'd need a SW44 at that point - you can just connect the eight cables from the four separators directly to your receivers.
If you do intend to have HD, you have several options. Again, you'll need a 20" elliptical dish skewed to pick up the 82 satellite. With this kind of dish setup, you have at least three different options for LNBs:
1. You can add a second single "dual" LNB and feed all four cables into a SW44. You can then "stack" an additional SW44, allowing you to connect up to 8 receivers. You don't need any separators with this kind of setup.
2. You can replace the single "dual" LNB with a "quad" LNB, which is essentially two LNBs in a single housing with four cable outputs. A "quad" LNB has a built-in SW44 switch. Only problem with this setup is you'd still need at least one more SW44 to feed more than four receivers (and you may also need a non-"legacy" quad LNB, since "legacy" quads may not allow switch stacking). Advantage is that the cost of quad LNB probably cheaper than buying another single "dual" LNB and SW44;
3. You can replace the single "dual" LNB with a twin "dual" DishPro (NOT DishPro Plus) LNB. This is like a quad LNB, but with only two cables out (i.e. it has a built in SW21 switch instead of a SW44). You can then feed the two cables into a DP34 switch, which is a two-in, four-out stackable switch, i.e. you can add up two two more DP34s to feed up to twelve receivers. Advantage of this option is you don't have to drill any more holes through your walls.
Most of the above hardware is NOT available from BEV, since they don't support it. I've always had good success getting it on ebay. Alternatively, if you're in a bigger centre, there's probably four or five independent satellite TV suppliers that would have the hardware.