I have been running into this issue in several modern versions on Fabric. 1.19.2, 1.20.1, 1.20.2 being the versions I have mostly been working with.

I do not know if it is directly Controlling or if there is a conflict somewhere. So it there is any known incompatibilities on Fabric please let me know. Or if something has changed and I need to do something to add the second key now.

When I try to set a combination (i.e. "Shift + G") it sets the first key to register and sets it to that. So as soon as I press shift that gets locked in and the "input" phase closes, not taking the second key.

Modifier key bindings is a feature added by Forge and other mods, fabric does not have it built in.

Do you have a mod installed that adds them?

Not that I am aware of. This is my first time messing with Fabric. I do not know what mods could add that function >,<;

It looks like a mod called Amecs might add that function. I'll test if they work together in a min.

Negative Ghost Rider. Amecs does add the capabilities to do modifiers, but it does not work with Controlling as the interface.

Yea, that would be this issue #77

Unfortunately I don't know if a solution for modifier keys on fabric, and there is nothing wrong with Controlling itself, so I'm going to be closing this issue.