Requires iOS 5.x.
Not yet compatible with iOS 6
Merge improves Messages by combining all messages by contact instead of by address. Instead of each contact’s phone number or email using its own conversation thread, Merge combines them all together and marks a line to show you which number or email is being used.
Merge also offers a quick way to switch the address you are sending to. To change to a different address in a conversation, hold down the Contact button or contact icon on the iPad to choose another address to send to. Holding the contact’s name down in the New Message composition view allows you to switch addresses also.
No settings or options to configure.
See larger Screenshots below..