Waiting for Astaro to chime in. This is not an Amazon EC2 limitation; it has nothing to do with it.
I am easily able to create Alternate IP's (or eth aliases) in Amazon Linux on the same physical interface. And this is extremely important when having to separate VPN connections.
I figured out a way around this, though I don't think this is a reason not to allow Alternate IP's to be created.
I made up an IP that I wanted in the Network Definitions and used that in the VPN configuration as the LAN IP instead of the real Private IP of the router.
Then I had to create a DNAT rule that when traffic destined for this fake address was coming in, to change the destination IP to be the real Private IP of the router.
And on the other side (source side) I created SNAT rule so that traffic coming from the device would be masked as coming from the IP I made up.
Just to be sure, you need that additional address for your VPN connection, right? Whats the reason for that?
AFAIK there is no way to use addresses assigned manually within an EC2 instance, you have to use DHCP for that, thats a EC2 limitation/security feature...