Hi,
From Kentico point of view you have to setup
web farm synchronization.
The rest - the load balancing and fail over is managed by third party tools and many customers are using many different tools for it - it depends what is bets for you. You just need to configure them correctly to point to correct Kentico site in given IISs and that's it.
Regarding licensing - you need to have an additional license for each server in the web farm. For more details please contact our sales department (sales@kentico.com).
Best regards,
Juraj Ondrus