You are talking about two different sketches. One on XY, and one on XZ.
The construction line and its end points exist in the first sketch. The do not exist in the second sketch. Therefor you cannot select them when you are working on the second sketch.
If you want to use something from the first sketch while you are working on the second sketch, you need to use the Project tool, to capture the item from the first sketch and create a representation of it in the second sketch.
Watch how I use the Project tool while I'm working on my second sketch, to capture the end points of the construction line from my first sketch, and create purple points in my second sketch. Now those purple points in my second sketch will always stay with the end points of that construction line in my first sketch. So in the second sketch I can attach other things to those purple points, and therefor create a dependency. Items in the second sketch will move according to changes in the first sketch.
Video: https://knowledge.autodesk.com/community/screencast/466f6ac9-ccb3-41a2-8cf8-f322af1a7040
