The two writers will have two different new root nodes which need to be merged, and that could trigger further merging of lower level new nodes recursively. It

The two writers will have two different new root nodes which need to be merged, and that could trigger further merging of lower level new nodes recursively. It becomes a merging of two subtrees. The subtrees can be quite different if node-splitting happens during inserts by the writers. It quickly becomes a complicate problem.