Can the active and passive copy of a mailbox database reside on the same server?
This article says no: Database copies can be created only on Mailbox servers that don't host the active (mounted and in-use) copy of a database. You can't create two copies of the same database on the same server. http://technet.microsoft.com/en-us/library/dd979802.aspx
This article says yes: In Exchange 2010, you can host both active and passive database copies on the same server when the server is configured for mailbox resiliency. http://technet.microsoft.com/en-us/library/ee712771.aspx