Sam Tsui and Kurt Hugo Schneider must love us a lot. They've been here a bunch of times — always entertaining, always endearing. And in July, the American singers who climbed to fame through YouTube will return for another concert. To date, the duo have covered all kinds of pop tunes, adding medleys, mash-ups and original tracks to their repertoire. Individually, they're a talented singer and a producer, but together, they're kind of magical.