With two EC2 Craft instances, the Control Panel session keeps expiring
Easiest solution: if AWS supports "sticky sessions" at the load balancer level, then you should enable to to ensure that the user gets sent back to the same EC2 instance that their session was created on.

Next easiest: create an NFS share and use https://secure.php.net/manual/en/function.session-save-path.php on all EC2 shares so all EC2 instances read/write to the same place.  You'd also want to set the Craft appId and validationKey config settings to be consistent across all of the EC2 instances.