How do I teach my toddlers how to share voluntarily?

I'm a father of 23 month-old girl/boy twins who are just beginning to develop morality. I'm also an atheist with strong Objectivist leanings. I don't want to teach my children that they shouldn't commit a particular offense because God is watching them as that will instill only fear of the unknown in them. What should I do instead? The twins also fight over particular things (e.g. toys, books, plastic containers, etc.). Too often I find myself trying to keep the peace with the one word command of "Share!". Forced sharing offends me but I find myself using it with the children because their understanding is limited and because it's easy to use. What might I do instead?