It's really easy. Get 2 (or more) machines that are well connected. I have 2 root-servers at server4free.de. Install tiki on both machines, but the mysql db only on one. Direct the mysql connection of the other machine to this one. In Admin->General, set to save the Session in the Database. Then create multiple A records for the Servername (e.g. redflo.de) with the different IP's in your DNS zone file.
redflo.de. 600 IN A 184.108.40.206
redflo.de. 600 IN A 220.127.116.11
We (ohertel and me) have set up a very fast 3-machines cluster: dupli.redflo.de. You can see in the "who is online" module how many people are on these 3 machines (london152, butan180 and florenz033).