Well, really the only right way to do this is to use custom media types, one for each document. That is you would have something like: application/vnd.company.xsd1.xml and application/vnd.company.xsd2.xml to specify different Accept or Content-Type headers. Then your RAML would have something like:
If it is a POST/PUT/PATCH with a body being sent in… same thing. Though, to be honest this sort of two or more different document types should only be for responses. I would suggest different resources for sending in two or more different body types on a POST/PUT/PATCH request.
In the above definition… the API consumer would pass in the Accept header one or the other media type to indicate which they want back.