Check correct pair member

I haven't looked deeply, but this looked to be a simple typo unless it's checking one both times because of a type invariant. If that is the case, and the check is correct, then a comment and/or code refactoring to call attention to this would likely be a good idea.

kitty/layout/splits.py

@@ -566,7 +566,7 @@ def add_pair(p: Pair) -> Dict[str, Any]:
                 ans['one'] = p.one
             if isinstance(p.two, Pair):
                 ans['two'] = add_pair(p.two)
-            elif p.one is not None:
+            elif p.two is not None:
                 ans['two'] = p.two
             return ans