Create a lobby, share the join code, race to solve rounds first.
How duels differ from solo
Rounds don't follow solo's "no two cities share a country" rule -- each round is picked independently, so the same country can come up more than once in a match.
There's no Reveal button. A round nobody solves before the timer runs out is shown to everyone automatically instead.
Report Round exists here too, but it only skips the round once every player has reported it -- not just one.
Duels don't feed the solo leaderboards -- it's a separate mode, scored by first-to-N round wins instead.