Photo: Sean Chee If you like opera, how about two at once? If you don’t like opera, how about two at once? In The Comet/Poppea , Yuval Sharon, the ever-inquisitive director and rethinker of the art form, presents double the bill in half the time. Even the audience is bifurcated, moved out into the wings of the Koch Theater’s stage, leaving the auditorium vacant. There are some lofty concepts whirling around this production, and the actions (both of them) revolve, too, taking place on a slow-