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 http://prntscr.com/l8nwdq 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.