At the risk of being pedantic 🙂 you don’t create a forest between two systems, you create a forest and join them to it.
However (being in the middle of creating my own forest), I’d first have to ask – what will it gain you?
If FTP over the VPN connection is slow, adding more administrative traffic isn’t going to help.
What you might consider is using secure ftp (FTP over SSH) for the file transfer instead of a VPN.
If any of that approach interests you, write back and pursue it some more.