Question
What should be done to prevent the cedar from getting two main branches?  My cedar looks like it has two peaks.

Answer
It sounds like the tree is a forked tree. This is genetic and IF the tree is young you can cut one fork off and the other will become the main trunk. The tree will have a lopsided look for a couple of years or so but will grow back to a full shape. OR you can let it be and the tree will be ok with two terminal leaders.