Throwing Daggers lets you attack up to 2 different monsters within 1 tile. You cannot attack the same monster twice.
I’m not sure about how Advantage works with an attack action that lets you attack multiple monsters, but a careful reading of the rules leads me to believe that your Advantage holds for the entire attack action.
Rule book, page 8 wrote:
While your Hero has Advantage, the next time he or she makes an Attack, roll the die twice and use the higher result. Then, discard the Advantage marker.
Rule book, page 10 wrote:
Making an Attack
For each enemy a Hero’s power or a Monster’s attack targets, roll the die and add the power’s attack bonus.