Thank you for the quick reply! My sftp XSD (using 2.0.3.RELEASE?) doesn't seem to have an order attribute defined, and only setting order="2" on the service-activator didn't seem to have the desired effect.
However I could implement the same idea with two channels and a wire-tap like so:
The wire-tap copies the message in preSend, therefore the "tap" channel will receive its copy first - that must be our outbound channel adapter.