I use the multi-track or multi-take approach and it works fine like on sketchfab: Clown Fish Low Poly Animated - Download Free 3D model by Denys Almaral (@denysalmaral) [af7ba2a] - Sketchfab
But I’m using 3ds Max, you can download my model and do some tests if you like, import and export in blender, and see if preserve the multi-take info.
In the other hand, I never could make it work with multiple animation files in Sketchfab viewer.