Hi,

First NIC is for management (network_interface) most interfaces defaults to this interface (api_interface,tunnel_interface,etc). This is an interface used for API request, user access, VIP creation and have an IP associated with it.

Second interface is for neutron external network (neutron_external_interface), this interface will be used by OpenvSwitch and will be attached to br-ex.

If both interfaces are the same, when the interface is attached to br-ex, will loose all communication with the host, as the IP address on it will be removed because is fully managed by ovs and not the host.

For the nodes who need the second interface, it depends of what your network architecture will be. If will use default tenant network + public, only network nodes will need neutron_external_interface, while if using providers networks also compute nodes need it.

In you case I guess control also contains network, so this node will need the second interface when using tenant networks.

Regards