Use server clustering on your front end server. This will ensure hardware redudancy.
Clustering provides high availability, but not redundancy. I think what you want to do is have both servers available for front end OWA services and load balance them instead. Typically you cluster the mailbox backend servers and load balance the front end OWA servers. Here is a helpful guide for you.
With any front end web applications, you would want to front it with a load balancer (F5, Radware, etc) and then use a VIP to represent the group IP address configured on the load balancer. This group IP (configured with DNS) and SSL certs will then be redirected to the server configured within the group. Redirection can be done by load, round robin, and various other methods.