You can't just do that (unless your filer names are site1bak and site2, which I doubt they are). You have to set up a connection name in snapmirror.conf. For example, put something like this in the beginning part of snapmirror.conf
# "connection" is just a name I chose. You can make up your own name...
connection=multi(site1bak,site2bak) # assuming site1bak and site2bak are DNS or host file defined addresses. If not, use actual numeric IP addresses
connection:vol0 site2filer:vol0_dr - 0 * * * # of course substitute the appropriate values for your configuration.
Hope that helps. Read up on snapmirror connection names if you need more clarification on how to use them.