Can't challenge anyone?
I am using duo lingo in iPhone 5 with all current updates.
My partner and I wish to challenge each other, but we have not been able to make it work.
Do you or your partner have a Game Center account? That's the only way, 100%, that you and your wife can set up a game for each other. When you're waiting for a duel, Duolingo randomly chooses any other player that is also online waiting for a duel and pairs you together. It's possible to get paired up with your partner using this way, but it's a very slim chance. If you don't have a Game Center account, then it's almost impossible.