For these purposes, I would suggest you to combining multiple maps under a Mix
map as described in this "Density by texture
mode (and therefore also the Paint Area(s)
) on such a surface would be possible to use in case you'll "split" the distribution into two Forest objects (and this way each of them will "project" on one side). In case you'll decide to use this work-flow, please be careful with local coordinates system of used "base surfaces". Attaching screenshot.
Hope you'll find it helpful.