We currently run magento enterprise on a single EC2 AWS instance with an ELB out front and MySQL RDS.  We are looking to move toward a distributed load balanced environment with two magento EC2 instances so we can handle downtime and more volume.  Our ELB is setup to do round robin distribution.  

My primary concern is what to do with the admin.  From what I've read it sounds like this should run on a single server to ensure the cron and observers only execute in one location.  Some put it on an alternate server instead (we don't have that luxury).  What are the recommendations for only running the admin on one of the two servers?