black walnut tree
Without a photo, I can't make an accurate assessment. But cabling, or otherwise shoring up the tree, is not generally recommended. Read here:
Trees with two leaders are inherently structurally unsound. They frequently have included bark and are prone to fail. Trees that threaten lives or property are called hazard trees:
You should have your tree assessed by a certified arborist:
It may be easier to remove the tree when it is 40 feet tall than when it is 75 feet tall.