Codominant stems are two or more stems that grow upward from a single point of origin. Because of this configuration, there is little direct connection between the wood of these stems. In some cases, bark from the stems is captured in-between the stems and this makes the attachment even weaker.
The best way to prevent this situation from occurring is by pruning a tree while it is young to improve its structure. Pruning should start a year or two after planting and continue as a tree grows. Having a young tree structurally pruned several times can nearly eliminate this problem. It also costs far less to structurally prune a small tree than to treat a large tree to remediate codominant stems.
If your trees were not structurally pruned when young, you may still be able to structurally prune them, but more likely they will need a structural support system to reduce the risk of failure. Structural support systems are steel cables or steel rods that are installed between the codominant stems to reduce their movement. Early pruning and structural support systems are two methods of increasing the strength and longevity of your trees.