I think you can try with:
rsync -azvu -e ssh user@host1:/directory/ user@host2:/directory2/
(and I assume you are on host0 and you want to copy from host1 to host2 directly)
If the above does not work, you could try:
ssh user@host1 "/usr/bin/rsync -azvu -e ssh /directory/ user@host2:/directory2/"
in the this, it would work, if you already have setup passwordless SSH login from host1 to host2