Using two IP addresses

I am currently using two Public IP addresses in our company, each one from diferent ISP. The main reason is to have a back-up line in case one of them drops. I have succesfully configured a load balancing.
My point is, there other members of our company which work outside of our LAN, so they do connect to our server (mail server) using the Public IP. Now that we have two of them, how can I configure their connection to use the other IP in case the other one is down?

Any help or direction?

Thank you Toni