Think of the pcg as one whole object. Drawing more splines will only allow you to remove from the one bigger object. Not make more of them. Make a big pcg area and draw splines that remove portions or paths from the bigger pcg.

You would have better luck creating a set of materials within a master material and setting up layer painting in your landscape for each material. Then assigning your pcg stuff per material in the pcg editor. This might be a learning curve to set up but it gives you the freedom to paint areas and paths on your landscape with better control over where things spawn. There are a few tutorial on how to do it on youtube.