Hi,
Please check these settings.
You can store session data in a database (dynamo db may be ) as it will give you a centralized location where all your data will be stored. In this case even if you loose any ec2 instance sitting behind your ELB you will not loose the session information. For more details on this you can go through a white paper named “AWS_Cloud_Best_Practices” released by aws. From page number 7 of the mentioned white paper they have discussed stuff similar to the kind you are looking for.
Or you can check this one
Thank You