Co-dominate stems often become a problem in mature trees. Co-dominate stems, or “double trunks”, arise when the terminal growth point on a tree is broken or cut off. The tree then sends up two or more new leaders which compete for dominance throughout the tree’s life.
Problems arise when these multiple trunks grow large in size. They are prone to failure (to varying degrees depending on the species) because as the trunks increase in size there is a smaller percentage of each trunk actually connected to the main stem. They begin to press against one another, trapping bark between the two leaders. It may look like they are attached to each other at this point, but in fact there is only bark between the two trunks. Windstorms and heavy snowfalls, among other forces, can cause the failure of one or more of the trunks.
I see quite a few trees with multiple tops while working in the Seattle area. This situation can often be remedied, especially if it’s caught while the tree is young or within 10 years or so of losing its top.