Would you provide information for Magento EE production with HA.
I propose Active site => web host x 2 with load balancer, MySQL
Stand-by => web host x 2 with load balancer, My SQL
Shall I need stand-alone Redis server on both sites? or just installed on web host with Magento?
in this situation your redis cache should go to a separate instance with replication. as you keep sessions and cache there it mustbe always awailable for live and fallback resources.