Second NIC not showing when creating a new virtual server or trying to change Ip address

Hello, I cannot seem to find a proper way to add my second NIC in my server.
My host is a XEN Server virtual machine running Ubuntu 14.04 so my NICs are virtual adapters both in the same subnet and both behind a NAT and assigned a different external IP address from my provider.
While i can add my second interface eth1 successfully and activate like in the image i cannot find a proper way to assign this to a virtual server.
In details when for example i try to Change IP addres the only interface i can see in the dropdown box of Ipv4 is the shared address of all servers.
Any help will be appreciated.

No one ever faced a problem like this ?

Screenshot doesn’t show up and your description of setup doesn’t make sense, to me anyway.
You have a XEN virtual machine with 2 virtual nics? Or the host machine has 2 nics and you want to assign nic2 to a XEN vm?