There are a couple of ways to do this.
If you'd like to rely on DNS to handle events, it is possible to create zones that will act as a round robin.
However, if your 2 servers are located with 1 hosting company, then you could do this on the router level and institute a failover procedure.
Also, third party DNS providers such as this one:
http://www.ultradns.com/services/sitebacker.cfm provide you with an easy to use and easy to manage method of reducing downtime.
However, if you really need 100% uptime, I highly suggest getting the help of a qualified sysadmin that you trust and then make these decisions together.