There are a lot of pairs of names (mostly names of people -- I'm weak on people) that seem to have only one slot in my mind. When I mean to mention one, I often have to think, or even look it up, to avoid coming up with the other. Here is a list I have collected over the years. I invite you to comfort me by confessing to your own mental fogs.

Amazon, Ebay
Barry Finn, Shay Walker
Bill Staines, Stan Rogers
butane, propane
Ceylon, Madagascar
Cole Porter, Noel Coward
Cyprus, Malta
Edna St Vincent Millay, Elinor Wylie
Figaro, Tivoli
From Here to Eternity, The Naked and the Dead
George Gershwin, Irving Berlin
Iraq, Pakistan
Jack London, John Reed
J. D. Salinger, John Updike
Jean Ritchie, Margaret MacArthur
John Betjeman, Philip Larkin
Max Eastman, Max Lerner
Peter Drucker, Peter Viereck
Phil Ochs, Tom Paxton
Saskatchewan, Manitoba
Steve Allen, Woody Allen
Tropic of Cancer, Tropic of Capricorn