Here is an article on how to associate multiple GRID engines with a single XenApp instance: http://xenserver.org/blog.html?view=entry&id=93. This was tested with a K2. Note that this is not officially supported by Citrix or NVIDIA at this time, but illustrates an interesting option of how to better leverage a single XenApp instance within the XenServer host environment.
Note: The article was updated on 2015-May-20 with much better images so you can see the frames in much greater detail.