The VM hubs are sometimes not very good at handling two consoles on the same LAN as they use the same ports and the UPNP doesn't always direct the correct traffic to the correct console.
One suggestion which works for some people is to put one of the consoles in the DMZ and setup port forwarding for the other one. When using port forwarding on the Hub 3.0 it is important to note that the firewall on the hub needs to be enabled in order for port forwarding to work.
If that doesn't work then another possible solution is to try connecting one of the consoles via Ethernet and one via wireless as that has worked for others.
The last option is to put the hub into modem mode and get your own third party router which can handle UPNP better than the Hub 3.0 (that is what I did and have had no issues with NAT on my PS3 or PS4). There is a section of the Xbox forums where they give feedback about routers and mention if they can handle multiple consoles:
Looks like the MS have changed the Xbox forums and the nice router section they had before has gone and the forum seems a bit harder to browse for info IMHO. If you know which router you want information about you should be able to search and the forum will give you relevant posts.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong If my answer solves your problem please mark it as helpful as it may help others My setup: Vivid 200 Optical fibre with Superhub 2 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.