The trunk is cracked / the tree split down the middle — what should I do?

A vertical opening you can slide a coin or a finger into. Two stems separated at a V-shaped union with a gap that has visibly widened. Bark peeled back along the seam, sap or water weeping from it, and — the decisive one — the two halves of the canopy drifting apart independently when the wind moves them. The stem has already partially failed. Whatever wood is still intact is now carrying the entire load the whole trunk used to share. This is different from a wound the tree can seal around: a shear crack propagates, lengthening a little with every wind cycle, so the trend is one-directional.