Exchange 2010 DAGs and licensing requirements

A very quick question that I can't seem to find an answer to online:

Do I need an "Application and Database" license for each member of my DAG, or just one for each member of my DAG that I back up?

e.g.  I have a DAG with an active server at data centre 1 and a passive copy at data centre 2.
I run all of my backups from the passive copy at data centre 2.
Do I need 1 license, or 2?

I currently have enough licenses for every member of the DAG, but renewal is coming up and I was hoping that I could drop one from the renewal quote.

Thanks