this function is like the "switch to previous preset" tool in krita 

it switches between 2 seperate CHOSEN brush presets,not merely the current and last used. 


the idea is that with this you can for example have a custom eraser preset that has different settings (more opacity, bigger size) that you can toggle to. You can sortof do this with switch to previous preset,but then you are limited to only 2 presets,as every new preset you choose removes the last used preset,which is not ideal for this workflow.

in effect it's the same as switch to previous preset,but when you choose a new preset it  replaces the current preset without moving that one in the place of the previous preset (which happens now)

another way to think of this is the COLOR BUTTON toggle,next to gradient button: You have 2 seperate colors to toggle between,if one is active the other is left alone,and doesn't get changed until you switch to it. From then on that preset is active and the other one is left alone.


You could very well provide a similar button as the COLOR BUTTON next to the preset button. The idea is that you can set a preset,toggle with button or shortcut,and choose another preset without affecting the first preset.





I realize the way I explain it might be a bit hazy but I hope it is clear what I propose:

A way to have 2 presets to toggle between,and a clear way to do so. The benefit is that you can set more accurately which preset is active without limiting to only 2 presets IF you want to use the toggled brush over and over again.