I did something similar (see Chocolate Cherry Popper Sweet Stout in my recipe drop down), but with cherry extract.
For a 6 gallon batch I used 4oz of chocolate extract and 4oz of cherry extract, both at kegging time. How I determined how much of each extract might seem a bit overkill at first, but I was hesitant just adding the extract to the whole batch and end up with too much/little, so here's what I did:
1. took about two pints out of the secondary and split it all into 10 glasses
2. separated 5 glasses to the left (the chocolate group) and 5 glasses to the right (the cherry group)
3. using a dropper I added 2 drops of chocolate extract to glass #1, 4 on #2, 6 on #3, 8 on #4 and 10 on #5
4. same thing with the cherry extract to the cherry group of samples
5. tasted each glass of the chocolate group and picked the best one
6. same thing with the cherry group
7. mixed equal amounts of best of each group into another glass and tasted that, just to make sure the mixture was spot on what we wanted
8. just for fun we played with different quantities of the remaining group 1 into glasses of group 2 but the mix tasted in 7 was definitely the winner
9. knowing the volume of each drop of extract, I calculated the amount of each extract needed for the entire 6 gallon batch and added them accordingly to the keg
We were actually a bit more anal than that and ate a salt water cracker between each taste
