rsync would be something like:
from slave1:
rsync -av data/pg_xlog slave2:/var/lib/postgresql/data/
Is that correct?
At this point I think your options are slim. If you are feeling adventurous, you can try doing the rsync with the slave running, then do a second rsync with the slave stopped or do it from the master after putting the master in backup mode (eg. executing "pg_start_backup('slave_backup')")
I didn't unterstand why doing RSYNC twice... sorry
Lucas