This was the problem I had that made me make my own splitter. Many trees have branches that reconnect, from first cousins marrying to remote connections between inlaws. Other splitting tools I have used did not handle this and you ended up with all the same people in both split parts. Splitter handles this by using a distance from the root person. If a person and their ancestors and descendants would end up in both trees then this person is assigned to both trees but their ancestors and descendants are only assigned to the tree where the distance is less.