If you wanted to use the interactive platform, just drop a variable on the x-axis . Then bring the second variable and hover it on the x-axis, JMP will tell you what will happen depending on where you drop it .
No, unfortunately this is not what I want. Your solution gives me a "nested" x-axis, i.e. in reality two x-axes next to each other. I want them stacked, i.e. both scales span left to right once and each curve is represented by its own axis. Actually, I ask for the very same concept as a secondary y-axis. It just seems not to exist for an x-axis in JMP.