Sumif Criteria Based On Combination Of 2 Cells

I have different products with different sizes becoming operational in various months & years. I need to sum those sizes that are operational in a given month/year. In the example below, I have

-product A with 100 unit size, operative since 2007/03
-product C with 200 unit size, operative since 2007/10

So in the table with months on left, and years on top,
from 2007/01 to 2007/02, value must be 0,
from 2007/03 to 2007/09, value must be 100,
from 2007/10 to 2008/06, value must be 100+200=300 and so on.

For this specific example, I tried the following:

Code:
E2 = D2&TEXT(C2,"00")
fill to E7
B11 = sumif(E2:E7,"