cfg exchng srvr to accept failed email

Our company has offices in two different cities. If the exchange server in Seattle goes down or is unavailable for whatever reason, how do I configure the phoenix exchange server to accept the email from the Seattle exchange server while it is down? And vice versa? Is this possible?

Is there a way to edit a dns mx record for one exchange server point to another email server which will accept any failed emails from the first server?

Thanks for any pointers to faqs, websites, etc.

Have a groovy day!