If both boxes are Humax they shouldn't work together, that is what I have read, the only configurations are one main and one or two mini boxes. Looks like you will have to call and speak to someone, I suggest at 8 tomorrow morning and maybe select the I am thinking of leaving option to get a response.
I got the same yesterday but decided to use them. The first I connected to the hub with the ethernet cable, sent the go text and it fired up straight away. The second one I connected by WiFi and the 7400 code came up, so I rung the help number gave them the box number and that came on. I'm well chuffed how they work in tandem. I'm so glad i didn't ring and ask for mini box as have still got 2 hard drives.
Thats good that it works but I needed the mini box without a hard drive as it is in my sons room and the v6 box constantly made noise that would waking him up but after 5 days of speaking to VM it is now fixed just waiting on a new mini box ps the new 360 box makes as must noise as the V6