FBX importing split animations with @ naming convention not working.

In my game, I have a rigged character with multiple animations. In the past, I've used the method that involves combining all the animations into one FBX file when exporting, and splitting the animations by frame manually on import. This worked fine, but it was inconvenient if I ever needed to add new keyframes to my animations, or add a new animation to my character entirely.

Recently, I came across this page in the Unity manual. It claims that animations can be imported using multiple FBX files, using the "@" naming convention. These are the steps I followed to attempt to implement this method.

1. I created a T-Posing FBX file called "archer.fbx", with no keyframe animation.

2. I created an idle animation and exported it as "archer@idle.fbx".

3. I dragged both into the same folder in my Unity assets.

4. I clicked Reimport on the "archer" model.

Yet... there's no animation. If I click the "archer@idle" file, I see the idle animation in the inspector, but there is no such animation in the inspector for the "archer" file.

I have tried dragging both FBX files in at the same time, and dragging in the base model first (followed by the animation). Neither worked.

I should also mention that each file has a "poseLib" action in the inspector, with a frame for each pose in my file's pose library. Could this be causing the problem? I'd rather not delete my pose libraries every time I export, but if it's the only way...

The only answers I've found for this issue are from 2013, and they all say to backdate the Unity editor. Did Unity just drop support for this feature?