ok i was wondering if i put 2 gpu heatsink together with the fins interlocking(dont know if thats a good word for it...) then would the fan i want to put on top of it still be effective with the heatsink?

In a word, no. If you'd like an analogy imagine trying to drink from a glass that ya covered with another glass flipped upside down, nowhere for the liquid to get out, same as if ya stick another heatsink on top of the other one with the fins interlocked, nowhere for the air to move across it to take the heat away from it.

if you just put a ~3000rpm 70mm fan directly on the gpu heatsink it will cool very well.