If you have the Summary definition 'Summary Total' line enabled (to get the column totals) then the rightmost column(s) - the measures being presented across the rows - should also total to the total of the columns.
However I have a feeling you already know that and you are actually asking a different question, so please feel free to prompt me for what I have failed to identify ...
Let's assume you have a summary with fields:
Then I assume you are showing the totals in the bottom row for measures a & b, and you want to add a cell to the right to show the total of these two totals?
If you create a new measure, and edit the formula for this so that it reads SUM(measure a)+SUM(measure b), then you should get the result you want.
You might want to experiment with colouring options to only show it for the total and not for the item rows of the summary - white text on a white background can work for this.