Q:

Receiver Module Second Data Pin

Many of the receiver modules (MX-05V) working with the Virtual Wire library (I would like to add a link here but it requires 10 reputation, so I'll do that later) have two data pins. However only one of them is used in the examples I have seen so far.
This picture shows a transmitter (top left) and a receiver (bottom right). Apparently the receiver has four pins - two of them are data pins. 

Image source
What is the purpose of the second data pin on the receiver module and how to use it?

A:

I would expect only one data line output from the receiver.
Looks like the manufacturer might have just connected the 2 data pins together. You could try using either pin to confirm that the two data output pins are simply connected together. 

