Dynamic Calculated Column based on field parameter selection

Hello, 

I created a field parameter to allow users to aggregate data by Column A or Column B.

Both of these columns store Month values (01,02,03, and so forth). 

Column A stores "Production" Months

Column B stores "Sandbox" Months

I would like to create a third dynamic column that displays Production Or Sandbox months

(depending on the users' parameter selection). 

I was hoping to use this dynamic column to join out to a dimCalendar table, so I can bring in additional information, such as # or work days within each month, etc.. 

I tried creating this calculated column, but I can't seem to get it to work.

Any suggestions?  Maybe I'm going about this the wrong way.