With multiple files in a file group, when does data start being placed on the 2nd file in the filegroup?
You'll want to preallocate the space on XWing_ExtraData.   At 10MB per autogrow, it'll induce a lot of external file fragmentation and delays whenever SQL needs to expand the file on the disk (performance will be better if you have a large pre-sized file up front).
Then disable autogrowth on XWing so that it can't grow any further otherwise SQL will continue to populate the file until you run out of space, triggering an 1105 error (filegroup full).
With regards to the original file, my preference is to just disable autogrowth via the dialog box (checkbox on the top of the dialog box or via ALTER DATABASE...MODIFY FILE...FILEGROWTH=0. You want to ensure that SQL won't attempt to autogrow the file if the algorithm can choose that file instead of XWing_ExtraData if both need to autogrow to accomodate the next data insertion.