Thanks for the question. At present this effect re-randomises the items so you will indeed lose your probabilities.
Another approach is to do this without effect, as follows.
- In the area rollout, add the Spline Include arear you want to use. Turn on Select models and pick all the grasses
- Turn on Density Falloff and edit the graph so that it drops to 0 pretty close to the left side.
- Increase the Include amount to pull the grass back from the edge of the spline.
Now we'll add the gravel.
- Add another new Spline area, and select the same spline from the scene.
- Enable Select Models and this time pick the gravel
- Check Falloff Effect > Invert curve. The gravel will now fill the edge.
Please find a sample file attached. It uses the new Forest Set feature in Forest Pack 8. This will mean that you can add as many additional splines to the scatter as you want without needing to go through the steps above for each one.
I hope that helps,