At the concert, Andrea Copado and Adam Kowal brought a sign which read, "You're ordained. We're engaged. Please marry us tonight!"
The former member of The Beatles unexpectedly invited them to the stage and pronounced hem husband and wife.
"I'm not sure that this is gonna stand up in a court of law," McCartney said.
"If Paul McCartney says I'm married, I'm married," Copado quipped.
Copado and Kowal also invited McCartney to their upcoming proper wedding in November in Chicago and they hope he can make it.