That worked for me (before I sprung for the DP44 and now have five slots ).
Make sure you have a receiver connected directly to the switch. The switch needs power and the DP21 doesn't pass enough to power the DP34.
Also, as seen in the diagram, make sure the single dish is on the "Dish 2" side of the DP21 and the DP34 is on the "Dish 1" side and remember, older receivers may not recognize the DP34-DP21 arrangement.