Helpless training dummy. Pinion, no ED, as few extra damage effects as possible. First number only, crits excluded.
First run planar focus set+black dragon set, second run black dragon set only.
The top end is more distributed than the lower end because of sneak attacks, which would have required more effort to exclude. If the black dragon set stacked, you would expect to see a drop of four damage from focus to nonfocus. If it didn't stack, you would expect to see a drop of 2.
count 875 804
avg 105.22 98.71
max 133 133
min 70 68
Nowhere near conclusive, but after an hour of watching the dummy, I'm personally ready to conclude the two sets do not stack.