I'm making a SPD 2013 custom workflow for a document set. I want to assign a task assignment and if a certain column equals a value also assign it to another user. Is this possible? Or is there a workaround for this?
Yes you can. You can do this through SharePoint Designer 2013. You can create a workflow for a document library. You can additional clauses where you can look at one or multiple columns, and pass conditions whereby they must equal certain values. It would essentially be if else statements.
Check the below screenshot.
UPDATE: Screenshot with two conditions.