First of all, there's no such thing as "Reagan's Eleventh Commandment", like you're claiming.
AND, Reagan didn't say it.
During Ronald Reagan's 1966 campaign for governor of California, Republicans established the so-called Eleventh Commandment: "Thou shalt not speak ill of any fellow Republican."
It was proposed by State Republican Chairman Gaylord Parkinson to help prevent a repeat of the liberal Republican assault on Barry Goldwater that laid the foundation for Goldwater's trouncing in the 1964 presidential election.
Just as Nelson Rockefeller and his East Coast cronies had branded Goldwater as an "extremist" who was unfit to hold office, so candidate George Christopher and California's liberal Republicans were leveling similar personal attacks on Reagan. Party liberals eventually followed Parkinson's advice, and the rest is history.
It was never meant to curtail rival candidates vying for the same office from taking on one another. Sheeesh!