The actor spilled the tea on his pal Josh Peck's "Good Guys" podcast ... saying he made the call to cut them out while filming the sitcom's pilot episode -- where his character, Uncle Jesse, and Dave Coulier's Joey had to watch baby Michelle.

It's a pretty hilarious scene, showing the 2 guys taking care of the little one while knowing nothing about parenthood -- but John says the taping wasn't a laughing matter.

Instead, he tells Josh and his cohost Ben Soffer that the twins were crying up a storm while hosing them off and changing their diapers.

He was told the other twin wouldn't cry when they were swapped out between takes, but it wasn't any better ... and John admits he made the call it just wasn't gonna work. He says the Olsens got the boot, and 2 other babies were used.

Of course, their firing only lasted a few days, with John admitting the new baby batch was pretty bad, too ... and as you know, the Olsen twins went on to stick around for the next 8 seasons of the hit sitcom.