My tree splits into two trunks and it looks like it's coming apart where they meet — what should I do?

Two stems of roughly equal size leaving the trunk at the same height, meeting in a narrow V instead of a rounded U. Down in the seam you see a dark, tight line of ingrown bark rather than a raised, wrinkled ridge of wood. Sometimes there's a vertical crack running down from the notch, a bulge of wood swelling out on either side, or a dark wet stain weeping from the notch after rain. Bradford and Callery pear, silver maple, and river birch often show three or four stems all leaving one point. The two stems are not actually joined. The bark caught between them prevents the wood from knitting together, so the union is held by a shell of wood on the outside with no bond running through the middle. This is a structural defect, not a disease, and the tree is usually in excellent health right up to the day it comes apart. It is the single most common reason a healthy-looking shade tree drops half of itself onto a roof.