Mode question for running two Cubes

[Hermonminer]

I'm running two Cubes both pointed to Slush's Pool. I have them set at Equal Priority Mode. Is there an advantage to setting to Primary/Backup Mode? Running in Equal Priority Mode gives me a hash rate of between 76-80 Gh/s. Will Primary/Backup give me that same aggregate? I would like to find out others experience before I experiment.

Thanks