Without a context, I'd say it means that there were at least two winners (it can't be "winer", but it could be "whiner" or "whinger"). A and B. Maybe A bought 100 lottery tickets and B paid for 50, and one of them was the winning ticket. The sentence is about A, one of the winners, and it mentions the other winner, B, who wasn't interviewed.
You can look up the adjective joint in a dictionary and learn that it means "shared by or affecting two or more " and joint venture and discover that it means "With individuals, when two or more persons come together to form a temporary partnership for the purpose of carrying out a particular project, such partnership can also be called a joint venture where the parties are 'co-venturers'." http://en.wikipedia.org/wiki/Joint_venture
But I'm just guessing. Always provide a context for such questions. Without a context, some things are just plain meaningless.