I assume that you are talking about "Grade letters". You can't, it's one map per course.
The whole treatment of scales and their relationship to values, for both aggregation and display, is (shall we say) sub-optimal in Moodle.
This was so critical for us that we have spent the last year or so writing our own custom version of the Gradebook that handles these scenarious. And, yes, it handles multiple value <=> scale conversions in the same course. Unfortunately, it will always be way too specific to be released as a general plugin.