Select passive or active copies for DAG

Hi,

DPM 2010
Exchange 2010 DAG set-up

I'm fairly new to DPM so please bear with me. 
When selecting the databases for protection, my understanding is that I should select only one database copy in the first screen then one copy in the copy backup screen.  Best practice say’s to select two copies. 
If I select the 'mounted copy' for the first one and then the passive copy for the copy backup I don't get the option for Synchronization frequency.  Instead, I only get the option to schedule an express full backup.  
Whereas, if I select the passive copy for the first one and then the active database as the copy backup the Synchronization frequency is available.
Can someone explain to me when this happens?
I hope I explained this well enough