Assign multiple Elastic IPs to EC2 instance

When I try and associate a second Elastic IP to a small EC2 instance the currently assigned Elastic IP is disassociated.

Do I need to use a VPC?

Classic EC2 instances can only have a single Elastic IP address associated with them. To get multiple IP addresses, you must use VPC and setup multiple network interfaces on your instance.

NOTE: sometime between 2012 and 2014 AWS changed so that a single ENI can have multiple EIPs assigned to it. This answer, and the question, are no longer relevant as a result.

To get 2 elastic ip's associated to one ec2 instance you need to provide a different route for the second ip. This involves to set a new ip rule which specifies what route should be used.

For example, there is an instance with two network interfaces (lets named eth0 and eth1), each one having one internal ip (172.31.4.255 and 172.48.55.23) which translate to his associated elastic ip's.

You need to specify for the ip of eth1(172.48.55.23) to take a different route:

 ip rule add from 172.48.55.23 table default

Then, associate his default route to that rule:

 ip route add default via 172.48.0.1 dev eth1 table default

And flush the cache:

ip route flush cache

You can dig for a "formal" explanation from this article