Outputs are low impedance and must only be connected to high impedance inputs -- never, never tie two outputs directly together -- never. If you do, then each output tries to drive the very low impedance of the other, forcing both outputs into current-limit and possible damage. As a minimum, severe signal loss results. 
Two resistors tie each input together to the junction of a third resistor, which connects to signal common. This is routed to the single output jack. The resistor values can vary about those shown over a wide range and not change things much. As designed, the input impedance is about 1k ohms and the line driving output impedance is around 250 ohms. The output impedance is small enough that long lines may still be driven, even though this is a passive cable.