Ok, I am a DirecTV / DirecWay Installer.
Short Version: no, you have to have 2 "home runs" from the dish to the receiver.
Long Version (Technical) No, the way directv cuts down on interference is to have 1/2 of the "transponders" using "Left-hand Polorization" and half using "Right-hand Polorization". Problem is the receiver "selects" one or the other by sending 13 volts for LHP and 18 for RHP therefore making it impossible to do both at the same time. In theory you could get lucky and use a single side power passing splitter and if you were watching 2 shows (both LHP or both RHP) you could do it, obviously not a good solution though.
1. Diplexers are for Cable/Antenna and satellite on the same line, the signal for the C/A side is lower (5 - 900 Mhz) and the satellite is higher (1000 - 2150 Mhz) with only the satellite side passing power (C/A is unpowered).
2. Splitters will not work, see "long version"
3. Multi-switch still needs 2 lines (one for 13 volts (LHP) one for 18 volts (RHP) )
Sorry, good luck though,
WTT