As far as i know this is not exactly possible..
You can however..
Continuously rsync the boxes together ( i dont believe FreeNas does any DRDB / block level replication)
To failover, you will need something like heartbeat or a proxy externally
FreeNas is a great product, but is not exactly supporting conventional/proprietary cluster services.. ( You might need to make your own)
You could also consider doing this from the application side, depending on what your actually using freeNAS for.. I believe with running something like VM or the windows equivalent you can
Hope this helps.