We highly recommend deploying more than one %BRAND_EXCHANGE_ONLINE_SHORT% 2013 server in your on-premises organization to ...

We highly recommend deploying more than one %BRAND_EXCHANGE_ONLINE_SHORT% 2013 server in your on-premises organization to help increase reliability and availability of hybrid deployment features. The best practice and recommended deployment configuration is to install the Mailbox and Client Access server roles on each additional %BRAND_EXCHANGE_ONLINE_SHORT% 2013 server deployed in your on-premises organization.