I did Istikhara on a marriage proposal and it turned out positive, however the boy also did Istikhara without knowing about mine and his reply was negative. What should we do?
Istikhara is not obligatory in every marriage. It is helpful if you are unable to decide whether the proposal is good or not.
The Istikhara which you did is for your intention, while the Istikhara which the boy did was for his intention. You need to wait till any change in the circumstances takes place, then if you remain unable to decide and you still need an Istikhara, then both should go for one joint Istikhara.
Wassalam.