In the very early stages of the Apollo program, it was even thought that only one astronaut would walk on the Moon, at a time.
The Apollo-LM toy models (1/96th scale) were even conceived that way with one astronaut inside the CP, one on the Lunar surface and one inside the LM.
But it didn't last too long before they realized it would be far more interesting, rewarding and safer (one helping the other) to have both outside.
Then of course, the re were the endless jokes about the CDR asking the LMP who was last to leave the LM not to close the door!

Another thing: the third astronaut who'd remain inside, is always in the pressurized environment; if there's a need for him to go outside in a hurry, it might take too long. The Shuttles always have 2 spacesuits for 2 astronauts who have trained for eva's (except for the flights where 4 astronauts go outside for the HST repair).
There are emergencies that require a man inside but also others that require a man outside.
Raoul