As you have discovered you cannot use rsync with a remote source and a remote destination. Assuming the two servers can't talk directly to each other, it is possible to use ssh to tunnel via your local machine.

Instead of

rsync -vuar host1:/var/www host2:/var/www

you can use this

ssh -R localhost:50000:host2:22 host1 'rsync -e "ssh -p 50000" -vuar /var/www localhost:/var/www'

In case you're wondering, the -R option sets up a reverse channel from port 50000 on host1 that maps (via your local machine) to port 22 on host2. There is no direct connection from host1 to host2.

As you have discovered you cannot use rsync with a remote source and a remote destination. Assuming the two servers can't talk directly to each other, it is possible to use ssh to tunnel via your local machine.

Instead of

rsync -vuar host1:/var/www host2:/var/www

you can use this

ssh -R localhost:50000:host2:22 host1 'rsync -e "ssh -p 50000" -vuar /var/www localhost:/var/www'

The first instance of /var/www applies to the source on host1, the localhost:/var/www corresponds to the destination on host2.

In case you're curious, the -R option sets up a reverse channel from port 50000 on host1 that maps (via your local machine) to port 22 on host2. There is no direct connection from host1 to host2.