[jholman]

My guess: Each owner says "it's your turn to feed it", and so the cow is never fed. And when the cow does give milk, each owner quickly takes the milk, claiming they haven't got their share yet.

It's like the human-scale version of socializing the costs/risks and privatizing the gains.

[collaborative]

That's exactly right. The original is "vaca de dos amos: ni da leche, ni come grano" (it rhymes as these things often do). Castilian proverbs are neat ("Refranero castellano"). Centuries packed into short sentences