Hi,
First let me mention, that's possible to "nest" Forest object (it's possible to pick the Forest object as a Custom Object in another Forest object).
Currently all objects which must be handled as an unique entity (as trees) have to be attached in a unique mesh (groups are supported, but for a different feature: to create clusters of objects that must be scattered together - like clumps of grass and flowers and each item is handled independently).
Anyway, described scene set-up should be possible by having copies of the Forest object (for trunks/branches & leaves) - maintaining the same General - Random Seed value. Hope that helps.
Best regadrs,