Diplexers will introduce approximately 3 dB signal attenuation, so use sparingly (3dB is roughly equal to a doubling of signal strength). We usually not recommend using diplexers in FTA installations. They can be used successfully, but if you can run an additional wire, you will have a much better signal especially critical for weaker transponders.
If you wish to connect multiple receivers, use a dual output LNBF connected to a multi-switch in place of the cable splitter along with the diplexer at each outlet.