Although preflight details may vary, you can use this workflow to separate the CMYK content from the Varnish content in the job.
The Duplicate Job preset is a Split node that sends all document pages to all branches
Once you have two copies of the job it's a matter of identifying Varnish content (typically defined as a spot color) and other content via two prelfight checks. There are a variety of ways to do this. The simplest would be to prefligth the spot color name
You can then use this preflight check with the preflight fixup to remove objects from the PDF
Build two profiles: One to remove Varnish Objects and one to remove all other objects.
The result will be two PDFs; one with only Varnish Objects and one with CMYK (and RGB, Gray, etc) content.