When you check the tick box A, post the result to another cell, say b20, when you check the second box, B, post that result to say B21, then where you want the result do the calculation =B20+B21. You can hide the cells B20 and 21 so the sheet looks neater.
I use a drop down list in some sheets I use for auto generating the Cetifying Engineers Approval numbers. In the drop down list are 5 names, the result of clicking on a name is 1,2,3,4 and 5, this result is posted to cell F3. Where I want the name to be posted has the following formula =if(F3=1,F11,if(F3=2,F12........etc) F11, F12...etc are the numbers to be posted in the result cell, these cells are hidden so it looks a lot neater. Hope that helps...