AWS multiple PEM for on EC2 instance

You will probably want to have a different PEM file for each developer that will be accessing EC2 instances. This is good practice so that when a developer leaves the company or you want to remove their privileges to SSH into an EC2 instance. Here’s the link to create multiple pen keys for EC2 instances:

http://utkarshsengar.com/2011/01/manage-multiple-accounts-on-1-amazon-ec2-instance/

Multiple SSL certificates for ELB

You can have two certificates on two different ELB ports. You CAN'T have two certs on a single port. Most folks would want to use port 443 for SSL traffic so using another port may not be an option. You can use SAN or wildcard cert that can handle more than a single domain and assign that to a single port. 

Certificates are managed using IAM. More details here:http://docs.aws.amazon.com/IAM/latest/UserGuide/ManagingServerCerts.html