Adding dubbed version of anime

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You can have multiple audio tracks in the same media file -- it's commonplace. Dual audio for anime is my preferred route. However, if what you have are separate media files, you would have to merge them using something like mkvmerge or ffmpeg. That is if, and only if, the media files match one another (e.g., both from R1 US DVD; both from the same source to ensure audio and video are synced).