The composerAction API can be used to put a button in maintoolbar OR formattoolbar, but not in both from the same addon.

I have a usecase (QuickText) where I need two different buttons/popups: One in maintoolbar, which opens the settings and one in formattoolbar to select a template to insert.

I could even think of use cases, where I need two or more different buttons in formattoolbar.

As suggested by darktrojan, I think a more generic toolbar API would be nice, maybe also fixing Bug 1583478 by implementing the label optional right from the beginning.