Methods of maintaining peace & unity in bi-national multinational and multiethnic states?

Answer 1

Focus on a common enemy.
Focus on a common goal.

But the devil comes out in the details. Take in the US for instance, the recent devil mongering on "Illegal Immigration". One side radicalized the issue to the great irritation of the other side, in the end, the "fence" has not been built (but most of the money has been spend). What I am thinking, on these issues that split along ethnic lines, compromise is seldom achieved, but more likely, either conflict, or total inaction is likely to result.

Answer 2

Find common goals in which all parties will benefit. I will leave it to you to define specific goals, which will vary depending what groups you are considering.