Again, I am going to use E* as a example, when they went to the Dish 500 size dish to be able to see both Birds, at first they went to a two L.N.B. solution with two cables coming out to a SW-21 switcher (kinda looks like your Voomer's diplexor) then one cable to the STB.
When I added the 61.5 Dish, I could not use another SW-21 (software would not support it, or so I was told), but by then E* came out with a dual LNB (switcher built inside) and then with one cable coming out to the SW-21 and 61.5 dish cable also going in to the SW-21 then one cable out.
Of course ,it depends if you have 2 or more STB, E* makes different LNB with 1 or 2 outputs, then then have those multi-switch units-like the SW-64, etc.