EV Design Question with Exchange 2010 DAG running over MAN
Since you are in a single Exchange org, plus a single DAG you really should use just one EV site. A couple comments:
1) Are they using Exchange in the cached mode? If so, and they should, then user access to archived data would be primarily on the local machine with minimal network traffic.
2) Alternatively, you could have a single Directory, but separate archive locations. Here you would have an archiving server, SQL server (for VS DBs) and storage in each site. During "normal" operations everything is separated, but in a DAG failover the archive traffice would go from the Exchange server, to the Archiving server (local) then via MSMQ to the server that hosts the respective archive(s) and on to it's index and storage. You would still have archiving across the network during a failover situation, but not normally.
I would probably recommend a single site, using cached OL. The amount of date being archived on a nightly basis should be within the MANs usaage levels.
just my 2 cents...