Assuming you mean to paint multiple trees in a certain place at the same time, I don't think you can.
answered Aug 31 '10 at 06:24 PM
Unfortunately this is not possible; Maybe someone could write a script to extend the editor?
It would be nice to be able to select multiple plants in one brush and determine the percentage of each plant being placed in one stroke. I'd be happy to have up to 4 trees in one brush.