Often trees with multiple trunks, or what we call co-dominant stems, are structurally defective and failure at this type of union is common.
Most major tree failures occur as a result of a co-dominant stem failure.
A Total Tree Care Arborist will inspect your tree, assess it for structural weaknesses and then

recommend the best type of cable bracing design given the circumstances. Cable Bracing Systems are installed to offer supplementary support between two or more stems. We use and recommend 'Cobra' Cable, the latest innovation in tree support systems.
This modern approach to bracing doesn't require wounding the tree like traditional steel systems. It's synthetic construction along with built in shock absorber makes for a much less rigid, more dynamic system.
The dynamic bracing system reduces sudden stress, thus preventing branches from breaking during storms and hurricanes. The static loadbracing system prevents the branch from falling.
Optimum effectiveness can be achieved by installing the bracing system 2/3rds the height of the stem above the crotch.
Only Cobra Cable attaches to the tree with a unique splice (see bottom right pic) that self-adjusts in girth as the diameter of the stem increases with growth. Beware of Arborists offering cable brace installations with similar but cheaper materials that attach to your tree with a "Locked" splice. As your tree grows this will "choke" the tree, damaging the cambium, and possibly weaking the very stem you were trying to offer support to!