Dynatrace automatically adds a second axis to your custom chart if there are different underlying units for these measures. So if you add two measures that are based on percentages, Dynatrace will assume that they are comparable and show them with a shared axis:
If you add another measure with a different unit, Dynatrace will add a second axis:
I'm not aware of a way to force the creation of two separate axes. In the "planned enhancements for custom dashboards"
thread there is a mid term improvement listed though:
So this is probably already on the roadmap. You can find more information about creating custom charts in the documentation: