There is information in the upstream how to sort the curves. The nodes do the right thing, just not sorting based on position along given axis or other rule.
A good alternative is to extrude the curves into polygon strips and merge the mesh objects. Then using a group object with bounding object attached to it you can work only with the edges representing the curves.
You can achieve the same with wiring bunch of nodes to sort the point cloud based on position along axis or other rule, but i think the proposed solution is much simpler.