Looking at only your very specific question, I think that it is asking if there is a way to make it so changes made on one drawing ( the "Subfile") will also be automatically made on another drawing (the "master"). I don't know of a way to do that.
My comment was a more general one which was based on an observation that doing that is usually a bad idea including losing organization/control over the master, potential issues regarding conflicting changes, and all in all usually runs contrary to the reason for even having two separate files, and usually against the very definition of a "master" copy. Of course, there are always exceptions, and your situation might be one of those exceptions. .