Secondary Brush Tool

My request is for the Brush Tool, again! As I paint, I like to make frequent changes and tweaks to the brush engine as my needs and intentions for that brush develop; kind of like changing gears in a variably subtle or drastic manner. Saving the brush as a preset is not helpful because I adjust it so frequently. It would be very helpful to have the option to add an additonal or multiple Brush Tools to the tool bar, each with separate engines and mappable shortcuts, which could be rapidly switched between to grasp or fabricate a brush of a totally different nature without having to discard my previous settings before switching back again. In effect, I want to be able to suspend my current brush engine and use another without having to save the preset in the middle. I think many users could find employment for a Primary and Secondary Brush Tool option to very quickly alternate between the strengths of either brush without being slowed down by saving presets - or for helping to design complimentary presets simultaneously.   
The Mixer Brush is not a suitable alternative due to lacking Opacity controls. Currently I find using the Eraser on a transparency locked layer an almost convenient work around. 

Thank you for reading, I hope this suggestion ignites some consideration!

Oliver