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The spec should be state whether attaching and detaching a MediaSource with multiple media elements at once is expected to be supported or what error handling is required if it's not supported.
I believe that this is covered in section 2.4.1 Attaching to a media element. The first step checks to see if the readyState is not "closed" and indicates that error logic should be run in this case. If the readyState == "closed" then it is immediately transitioned to "open" which blocks any further HTMLMediaElements from attaching the MediaSource object until the element is detached.
You're right, sorry about that.