There are two ways of doing this depending on how you run DNS:
1. If you have your site running on another server (ie. same domain and site hosted on 2 servers) just write two sets of records in the DNS zone pointing to both server IPs. This is basic load balancing. Doesn't quite give you what you want.
2. Point your domain to zoneedit.com service and setup failover load balacing with the two server configuration as above. This will give you exactly what you want.