No can do - You CANNOT cascade 2 SW21's.
Your options -
Replace the 2 Dual LNB's on your Dish500 with either a TWIN or QUAD ( then you can cascade this with the 61.5 dish )
Or you can replace the SW21's with a SW64 (all outputs from all 3 Dual LNB's into the SW64).
Your options -
Replace the 2 Dual LNB's on your Dish500 with either a TWIN or QUAD ( then you can cascade this with the 61.5 dish )
Or you can replace the SW21's with a SW64 (all outputs from all 3 Dual LNB's into the SW64).