Adding a custom column for difference between two columns in pivot table

Hey everyone!

Is there a way to add a custom column to calculate the variance between two columns that will be dynamic? Here's an example. 

So the user would have to select 2 batallions to compare which are displayed in the columns. I need to create a custom column for the overlap. Is this possible? In this case, a third column will appear and in the 5K fuel trailer row, it'll show -4 since the second batallion has 4 while the first has 0.