Thanks for the interesting question. It's quite tricky, but I think you can achieve what you need just using a combination on include and exclude areas combined with falloff.
First of all you'll need 3 splines. A middle section (the ellipse in this example) that both areas share as an Include area. Because this area overlaps though, you'll need to exclude it first. Then outside of that you'll need a spline each to exclude the two FP generators. Here I've added one about and one below. You can then increase the Exclude falloff values for each Fp object to create a gradient in the middle zone
Note that if you are using other exclude areas in your object, you'll need to override the falloff effect for them by using the Falloff Effect parameters
This image illustrates how the 3 splines are used for the red Forest object.
And this image illiustrate how the splines are used for the green Forest object
A sample file is attached.
I hope that helps,