If you're using coax to connect to your tv, it's as simple as running the output of one of your receivers into the input of the other. Both receivers should support pass-through (all of mine do). - so when you turn off one receiver, you'll see the signal of the one that is turned on.
Otherwise you'll need some sort of physical switch, or (most likely) you'll just connect each receiver to another input on your tv since most tv's have more than one input these days.