Well, there is a "one cable" way to hook it up to enable both tuners, but I have a feeling if your parents need your help they likely wouldn't have gone down this road. See my example #1
here.
If they did go that route, I shudder to think of the potential issues of trying to add a diplexer to a line with a seperator- especially seeing as the other end of the single cable has to go into the DPP LNB.