Badda boomTo make a long story short. Add the two N-P-K's together, then divide by 2 (if you are using equal amounts of both products).
If you are not using equal amounts of each nutrient, it's a little more complicated. For instance, if you are only using 1/2 as much of "Product X", then you must first multipy Product X's N-P-K by .5 (because you are only using 1/2 as much). Then take the half sized N-P-K and do as above.
It's just math. Badda-bing!