two lines have the same start point and end point.

My work likes this: http://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=0ce0c1fb9846bf838191ca690811fe69

How can i separate them?

you have the same endpoint and starting point for both lines. if you use different variables the second entity that'll fix your issue