On the day of installation we were able to view recordings from either box on the other. Following the installation I renamed the V6 from it's default name to "Master Bedroom", I also used the website to copy the TiVO settings from the box that I replaced with the V6.
I'll be honest, we didn't use the feature immediately as the V6 box was empty so didn't need to view any recordings downstairs so I can't confirm if the issue occurred immediately after the above changes.
The problem now is the downstairs TiVO can view recordings from the V6 but the V6 can no longer view recordings from the TiVO. It shows the device under my shows but it's offline and we see error c423 when attempting to access it.
Any advice appreciated, I've contacted Virgin Media on Twitter and they aren't very forthcoming with suggestions.