Based on my limited reading of the spec it states:
	Quote:
	
	
		| It MAY refer to more than one EPUB Content Document, but an EPUB Content Document MUST NOT be referenced by more than one Media Overlay Document | 
	
 where  "It" here is a Media Overlay Document.
So the only way to include multiple media overlays to basically the same xhtml content document would be to duplicate that xhtml document under a new name and associate it with the second media overlay.
Probably better to just split it into two different epubs.