alter column varchar to nvarchar - will file group on full drive have issues?
So 1 filegroup with 2 files, one on each drive ?
In this case the writes will be distributed according to the space available on each file -
see the MS reference :- https://docs.microsoft.com/en-us/sql/relational-databases/databases/database-files-and-filegroups?view=sql-server-ver15
So the SQL Server engine should sort it out - however you might not get the best performance for this you would probably want a new filegroup with a number of equally sized files - but life gets complicated quickly and for most databases it is not worth the extra planning / management for the small percentage performance improvements available.
Only precaution I would take is to set the maximum size of the file on the nearly full disk to be equal to its current size, as you wouldn't want it to attempt an autogrow and for that to fail.