Hahaha...Hippie Speedball...
My anecdotal two cents:
Well, this evening I had much homework to do, so I drank about 2.5 cups of coffee within a two hour period. I need to sleep at a relatively decent hour tonight, so approximately 15-20 minutes ago I toss n washed two tablespoons of BKH Fu'u/half cup water. It feels as though my mind is kavanated but my body is kavanated/caffeinated.
So I guess based on this highly scientific trial I agree with dan. They neither interfere with eachother nor synergize. I was going to speculate that maybe parallels could be drawn with the ever popular GABA agonist alcohol, but alcohol also works on the dopamine system as well, so I guess that couldn't be an accurate comparison.