I was wondering... If I wanted to make two dense populations in the same area (for example pine trees and oak trees) will I run into problems of objects overlapping each other? If so... how do I get around this problem?
I have not had difficulty with this, but it takes some tweaking to get them to work. I hit random and play with the spacing until they work.
You might get some overlap. It only really matters when you're on the ground or just above the trees. From an airplane-type or reentering-spaceship-type POV it doesn't matter; you'll just see the tops and it doesn't matter whether or not the trunks or branches overlap.
Try using the invert of the first population's blend shader for the second population. :)
real trees also overlap and sometimes "share" the tree crown - so thats usually not a real problem unless you get that close, so that you can really see intersecting geometry like branches growing through the trunk of the neigbour for example.
At the beginning I also often tried to avoid overlapping plants and intersecting geomtry - but actually the result you get looks worse in most cases, to regular.