we do not have a simple API for this task.
You could write your own FileTransferClientParallel class that would internally hold more connected instances of our
FileTransferClient class. Your class would need a queue of files waiting to be transferred and it would distribute the work within 1,..,n FileTransferClients. Every time a client would finish one transfer it would be assigned another transfer. The parallel class would technically allow maximum n parallel transfers to be performed simultaneously.