You need to license every SFR module on firesight appliance then create a policy wich is applied to both targeted devices.
Appliance doesn't know if you have lost one sfr module, it just applied policy to both module.
I never tried that, but I think it the right way to do that.
In documentation, the only thing related to failover is :
"If you configure the ASA in a failover pair, the ASA FirePOWER configuration does not automatically
synchronize with the ASA FirePOWER module on the secondary device.You must manually export the
ASA FirePOWER configuration from the primary and import it into the secondary every time you make
a change. Upon failover, the module also loses all configuration on the device that failed over."
Cf : http://www.cisco.com/c/en/us/td/docs/security/firesight/541/firepower-module-user-guide/asa-firepower-module-user-guide-v541.html
But you should apply same policy to several module and all should be ok when ASA fails.
