Running Nixos with Gnome, and I have Piper with libratbag for my Logitech many button mouse.

I'd like to toggle a pair of keyboard keys up and down together via a mouse button. Hit the mouse button: both keyboard keys are pressed and stay pressed. Hit it again: both keyboard keys come unpressed.

Piper doesn't seen to support it, and I was looking at the Gnome accessibility settings without success.

Happy for suggestions!