Hello!
I would also be interested if someone knew of a built-in solution to deal with this :)
However intuitively, I think it is simpler to split the tile in two. This is the solution recommended in this blog (about Unity but more generally adressing sorting issues in isometric space):
https://connect.unity.com/p/articles-what-i-learned-from-trying-to-make-an-isometric-game-in-unity
Because at the moment you use that type of multi-cell objet, you cannot use y sorting at all anymore. I am new so I am not sure, but I guess there could be optimization in the built-in y sorting that would be difficult to get with custom sorting.
Actually I am not even sure there IS an existing sorting order that does what we want without splitting.
Tell me if you find something! :)