First of all you have to look at the comparables If most of your comparables show a 2 bay garage in your price range, that is what is expected so there is not any value added for such. If most of your comps don't have 2 bay there would be some extra value.
Just out of curiosity I ran some reports in Brookline. The median house price was $1,277,500.
So there have been 32 homes that have sold in the past month in the 1M to 1.5M range.
17 Had two car garages with an average price of $1,248,722
9 Had 1 garage stall for an average price of $1,152,143
6 Had no garages at all for and average price of $1,321,672
So what does this tell us... nothing!!
The best thing to do is contact several local REALTORS to provide you with a CMA or Comparative Market Analysis. A CMA will compare similar house.... meaning similar sq footage, similar location, similar age, similar style hopefully all with 2 car garages to determine the value.
Adding value for a 2 car garage does not seem to make sense in at least the median price range in Brookline. Remember determining value is not an exact science it is a little bit more of an art and requires an experienced agent to provide one.