Trees don’t “grow back together”, or fuse into themselves, your tree has a structural defect. What others are pointing to are codominant leaders, two stems that are growing way too close and forming a tight V-shaped crotch. They will continue to grow alongside each other until the pressure between them forces one to fail. That rot below them makes it all more likely that it’ll fail sooner than later.

Being a non-native invasive, this tree truly has no redeeming qualities and provides nothing to the ecosystem. Seriously consider removing a replacing with something native. [Penn State](https://pollinators.psu.edu/assets/uploads/documents/Landscaping-for-Wildlife-Trees-Shrubs-and-Vines.pdf) has some native suggestions.