System Mailboxes - Exchange 2010

Hi,

I have a question about system mailboxes.

we have 4 Mailboxes servers in DAG. I can only see 1 System Mailbox, 1 Discovery and 1 FederateEmail that are in 1 mailbox server and DB.

I see some post saying that you should have 1 System Mailbox per Mailbox server in the DAG. Is this true ? And if so how can I create additional System Mailbox per Mailbox Server ?