How to rsync files between two remotes which share the same host?
Normally you can't use two remotes for rsync, but because these two instances are on the same host you may use drush dd to return the filesystem path and use it as a source.

In example (syncing webroot):

$ drush -vy @remote.test rsync `drush dd @remote.dev` @self


or syncing just files folder:

$ drush -vy @remote.test rsync `drush dd @remote.dev:%files` @self:%files


This will work if you're using the same credentials for both.