[capink]

There are two solutions:

1. Create a New chain
   
   Create a new chain with a single action: "Search Using Templates", using the following Templates:
   
   

   program:
       if from_search('id', '#reading_status:"=Currently Reading"') then
           '#reading_status:"=Currently Reading"'
       else
           '#reading_status:"=TBR"'
       fi

   Note: Make sure the option "Template output is a calibre search" is chosen.
   
   
   
2. Modify your existing chain
   
   Modify you chain as follows:
   • Make sure in the first "Selection Modifier" action the option "Select first book in current view" is selected.
   • Add a second "Selection Modifier" action with this search:
     
     

     #reading_status:"=TBR"

     Note, however, that you must put this condition on it (Right click > Add/Modify conditions):
     

     template = program: from_selection('id')
     datatype = text
     comparison = "!="
     condition value =

The first solution is the simpler solution. But since your #reading_status is a composite column, I think the second might perform better. Experiment with the two and see what fits you better.

Edit: The two template functions from_search() and from_selection() are only available in Action Chains plugin. @chaley has since introduced a template function called book_values() that can replace from_search().