I have never used the server to server VPN option in NethServer and not sure if it is possible with NethServer to do multiple server to server VPN connections.
If the issue is that you need 1 or more green interfaces, and you only have 1 interface (being RED) there is an option to create 1 or more ‘dummy’ interfaces.
Have a look here: Create a dummy interface when you only have 1 NIC available
To wrap this up a wiki page has been created too (to install NethServer on a single NIC VPS, on top of a CentOS7 minimal image)
https://wiki.nethserver.org/doku.php?id=virtual_network_interface