This term is derived from the term cock block and is most commonly used while drunk. It refers to someone who is not attempting to split two people apart but join them in hopes of either a threesome or being able to hook up with the same person that night but usually has the effect of standard cock blocking unless the parties involved are completely shitfaced.
"Dude, your completely cock blocking that guy from that hot girl"
"I'm not cock blocking I'm cock sharing"
"wow you are really drunk"