Unless they are on the same ground. Then it would work.

I just had a closer look at my R909 and noticed it actually has 4 individual heatsinks; 2 per side.
Here's a well-lit picture from a review; the standoff in the center makes it difficult to see, though.


You can check out the beginning of that review for more angles (he moves it around a bit): https://www.youtube.com/watch?v=fgU1fUfEd3Y

I'm not sure if it makes sense to have chips on both sides of the PCB or if GekkoScience cools the chips from the die and from the PCB.
In any case, they do seem to group chips together; in case one heatsink was just PCB cooling, then 3 chips wold be sharing one heatsink.