Apparently, MRP is not the only one who confuses John Updike and John Irving. According to an insightful and touching (and even humorous) remembrance by John Irving himself, it happened all the time:
For a period of time—no longer—fans used to confuse the two of us. How could this have happened? Because we were both “John”? It was baffling, but I got numerous fan letters that were meant for him, and he got fan letters that were meant for me, and this gave us the occasion to write to each other—and send the misdirected fan mail to each other.
Read the rest of Irving’s musings in “Dear John: What I’ll Miss About John Updike.”