Have two communist countries ever gone to war against one another?

Yes. Vietnam invaded Kampuchea and ousted Pol Pot (thank God). These were not full blown wars, but China invaded Vietnam, then pulled back at their leisure. The USSR invaded Hungary, which was communist, to prevent it from going to the west in 1956. China and the USSR also had some severe border clashes in the 1970s.

No..since there have never been any communist countries..countries like Cuba who people call communist are not communist for the simple reason..there is NO DICTATOR running a communist country

Russia and Germany during WWII. (Of course, Hitler was not a communist but rather he was a Fascist. Russia was communist at the time but both were a totalitarian government.)

Vietnam invaded Cambodia in the 1970s. They were both commies then.

China and Vietnam;

Vietnam and Kampuchea

http://keenander.narod.ru/anders/and02.htm

this page is full of examples hope it helps