Your RCA multiswitch should do this for you. It sounds like you only plugged in one of the inputs though. You have to plug in both of the wires of the dual LNB to your multiswitch.

The way DirecTV receivers work is they can either tune to even or odd transponders. So, at any given time (depending on what you're watching) each LNB can be on either odd or even. The multiswitch locks one LNB to odd and one to even and then handles sending either odd or even signals to the receivers. So, if you only have one LNB plugged into the multiswitch, it is locked on either odd or even. Hence, you get signal on only half the transponders.

In short, both inputs of the multiswitch must be populated.