I think its a bad idea, mixing those 2 but then again that's how the breeds started out in the first place... 

Got pictures of the dog and parents yet?

mastweiler.muslovedogs.com/cm/

www.google.nl/search?client=opera&q=Mastwelliers&sourceid=opera&ie=UTF-8&oe=UTF-8#newwindow=1&q=Mastwellier