You can add another FilesMatch
section for those specific files:
<FilesMatch "\.js$">
Header set Cache-Control "max-age=608000"
Header set Cache-Control "must-revalidate"
</FilesMatch>
# CACHED FOREVER
<FilesMatch "(file1|file2)\.js$">
Header set Cache-Control "public"
Header set Expires "Thu, 15 Apr 2010 20:00:00 GMT"
Header unset Last-Modified
</FilesMatch>