That was exactly my line of thought. Another way to see it is to look at what the spring-balance feels. In the picture on the left it feels the lower rope pull

That was exactly my line of thought. Another way to see it is to look at what the spring-balance feels. In the picture on the left it feels the lower rope pulling down with 10N, and because it is at rest it feels the upper rope pulling up with 10N. In the picture on the right it feels exactly the same forces: 10N pulling on one side and 10N pulling on the other side. So the reading has to be the same. Yet another and perhaps the simplest explanation is that you pick up the left mass and you don't hang its rope over its pulley, instead you just hang it in mid air. Now we just have the rope hanging over one pulley, with the mass and the spring-balance on the left and the other mass on the right. Because everything but the masses is massless it's still in rest, and the situation is exactly the same as when the rope was attached to the ceiling instead of to the other mass.