Get a 1/2" auger bit and core check those for heart rot before hitting them with a saw.
I'd probably weaken the lower one down to a 1" strip of holding wood in the hinge and another behind the back cut (need to plunge the back cut), and then cut the upper with a flat face cut (also plunged), and then cut the back cut in two slices from the sides taking out the upper tree and the lower one's back tang at the same time. Whole thing goes down together over the lower's hinge.