I also use the same setup. I made a script using python transmissionrpc which watch the torrent pieces being downloaded and send them with sftp as soon as they

I also use the same setup. I made a script using python transmissionrpc which watch the torrent pieces being downloaded and send them with sftp as soon as they are available. Then on the receiving side it just creates a sparse file and fill in the pieces using dd (so no dependencies on receiving side). The sending speed was not very good. It could probably be improved by doing multiple transfers in parallel and using a less hacky way to transfers the data.