Two weeks ago, we were telling you about the first opera in the Washington National Opera season. Last night, this DCist heard the second one, a gala production featuring the company’s superstar music director, renowned tenor Plácido Domingo. The fact that he is singing in all three acts of this production, in alternation with working at the conductor’s podium for the other production, I Vespri Siciliani, is remarkable for a musician of his age. As…