I saw this recently in a video but couldn't make out how it was done. Where this guy had made clay brush his primary and smooth brush his secondary, then you can double tap a trigger to switch?
God that last reply is terrible of me, was in a rush while with in laws, lol.
IIRC you can use trigger on your off hand while selecting a tool to change the alternate tool. Does that make more sense? The secondary tool is indicated with a little green triangle.