So Measure Names (as generated by Tableau) is only a pseudo-dimension, and so don't have all the functionality of regular dimensions (such as it can't applied across sheets, or accessed in formulas)
One way round this is to use a parameter as a measure selector, and then you can use that field in both the Map and Detail. I've attached a quick example here.
Hope that helps, and let me know if anything in the attached doesn't make sense.
Select Measure Parameter.twbx 462.1 KB
Thanks, just what I needed !