There are two locked instances in Stormwind. Theres one that looks like stocks and theres one with a huge portal with a gate infront of it so you cant get in
1. Go to the one with a huge portal and a gate infront of it(its stormwind vaults on private servers by old town)
2. Now theres two ledges by the side of you to the left and the right go on the left ledge.
3. Now get somone to come to you and make them stand on the same ledge
4. Now right click there portrait and click duel