I'm trying to figure out exactly how the culture spread mechanic works. 

If you have two planets, each with with own Culture building, and both of them are affecting the same starlane, do the effects stack?  Does your culture spread down that lane at an increased rate? 

What I'm really trying to figure out is if it's more effective to put multiple culture buildings on the same planet, or if you can get the same effect by spreading culture generators throughout your empire.

In case the question comes up I generally play Advent.