Joe Biden and his wife, Jill, were spotted in the audience of the Tony-winning Broadway musical “Once” on Saturday night, spies said. “They have been wanting to see the show since the election,” but hadn’t had time to catch it, the source added. Afterwards the Bidens went backstage at the Bernard B. Jacobs Theatre to meet the cast. At the show Saturday night, the cast performed a special encore number, which had been cut from the production before it got to Broadway, to celebrate its first anniversary there. The Bidens caught the show just before it says farewell to its original stars: Yesterday was the last performance for Cristin Milloti, and Steve Kazee will also be replaced this week.