@nipundavid, what you are seeing in the sandbox is not a combination of the two glTF files, but rather adding the animation information to the scene. The loaded glTF sits in a scene created by the Babylon.js engine in the sandbox, much like you would see in Unity. When we create the scene with the dropping of a glTF file, we position the camera so that it is normalized with the bounding box of the model (so that the model is completely within the camera frustum).
The scene has all of the features of any other Babylon.js scene, which includes a full inspector where you can make changes to the scene. This includes importing animation like you did in your video. What happens is that the animation information from your selected file is imported into the scene and can be played on a mesh that has a matching skeletal hierarchy. So there is no assemblage of the original glTF file and the animation key data. They both exist in the same scene and can be played, retargeted, replaced, etc.
It is much the same as this example playground loading spinner where I am loading in a glTF file with some animations and a morph target. In the playground, I access the model to get the animation group from the model and add a second animation created in code to the animation group from the glTF and play both at once. In no way did I combine the new animation with the original glTF file and write it out, but instead both assets are loaded into memory where I have full control to do whatever I want with them. The original glTF stays unmodified.
This is what I was referring to in my previous post. You can create a playground (or a stand alone scene on your own web server) to create a scene and load in your mesh and animations separately and retarget as you need. However, if you need to combine the animation data into the same file as the mesh and send them all as one file to someone to view in a separate viewer then you will need to assemble them manually in your DCC tool of choice and export all of it at once.
Let me know if you have more questions about this. Take care!