You can not have a single IP on both machines at the same time, but you can quickly move it between them.
It seems you're after a load balancing/high availability solution.
I suggest you look into Linux-Ha - Heartbeat .
It's basically a daemon that would run on both machines, Webserver 1 and Webserver 2 and send "I'm alive" signals to each other, whenever the signal is not received for a specified amount of time, the other node is assumed dead (or made dead using a STONITH  device) and the active node takes everything over.
You can also pick active/passive nodes by hand. Starting from heartbeat v2, it's possible to monitor nodes at resource level, though the configuration is XML based and kind of complex, if you're going to look into it, I suggest using heartbeat-gui to configure it (Ubuntu has a package )
Novell has a great free ebook about this stuff .