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I think you don't require multiple email engines.Instead, you need to configure multiple incoming mail boxes.
I think that was the question. Multiple email engines are for higher loads. Multiple mailboxes for mail accounts. Multiple incoming mailboxes are not possible for OOB because the other mailbox is automatically disabled.
It only takes a small customizing to have two or more mailboxes active.
you could redirect the mails on exchangeserver side so that the from and to remains the same as original and query the to address in your rbe rules