"You get two for the price of one tonight," Nick Carter told the screaming crowd.
The New Kids on the Block – Joey McIntyre, 37, Donnie Wahlberg, 40, Danny Wood, 41, and brothers Jordan, 40, and Jonathan Knight, 41 – were sitting in front of a white curtain bantering when they launched into the Backstreet Boys's 1999 hit "I Want It That Way," an hour into the concert.
As the audience began to sing, the curtain lifted, revealing the Backstreet Boys – Brian Littrell, 35, Nick Carter, 30, A.J. McLean, 32, and Howie Dorough, 36, who finished out the song.
Cue the deafening screams!
After the big reveal, the musicians embraced on stage, and both groups continued singing. Then the New Kids ceded the stage to the Backstreet Boys, who performed another number.
"I want to thank those guys right there," a gracious Littrell said. "They are our mentors. We've been fans of theirs for a long, long time."
After performing their 2002 song "Shape of My Heart" while perched on stools, the Boys left the stage with some protest – "Bring them back," screamed some fans! – only to reappear at the finale when the NKOTB performed "Hangin' Tough."
At the end of the show, all took a bow and jumped off the steps on the stage as green and white confetti and streamers rained down from the ceiling.