That is not a split. That is 2 oak trees in a cluster of 4. They have grown together, partially encapsulated, and now crown growth and wind has overwhelmed the strength of the encapsulation. Over time, water will infiltrate and eventually cause decay and rot in both trees, leading to their ultimate failure... someaday... probably long after you and I have already decayed... something to keep an eye on, but not something to keep you up at night.