How to use landscape splines with world composition?

i’m using landscape splines for roads ands rivers but I don’t understand how to use them in a tiled world using world composition.
It seams I can’t place them while using the persistant level as current level.

If I select one tile and make it as current level, I can place one spline on this tile but then what?
How do I make a road or river to run across multiple tiles?
If the player is far and this tile is culled, then my road/river disappear.