The webpmux tool  can be used to manipulate webp files. It can be used to create an animation from multiple webp files.
In your example it's not clear what you mean by concatenate though. Do you want a still image with both images contained within it (e.g., one above and one below)? An animated webp can somewhat provide this, but the duration between two frames has a minimum. Otherwise to create a single frame you would need to reencode the images.
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