That doesn't make sense though, because the "but two" would still be referring to the group of single people. It doesn't magically switch the subject to everyone on the plane.
Yeah Fallen is right. Grammatically, the "but two" part of the sentence is referring to "single people". But the riddle is still a clever play on words.
Heres Another One... A Man And A Woman Were Driving In Their car When It Broke Down. The Man Decided To Go For Help At A Gas Station A Few Miles Back. He Made Sure No One Was In The Car, Rolled All The Windows Up And Locked All Of The Doors. He Went Off But When He Came Back, His Wife Was Dead And There Was A Stranger In The Car. No Physical Damage Was Done To The Car, So How Did The Stranger Get In?