Why would you be amazed that a single box picks up a single channel?

Yes, I do find that amazing. Basically the STB needs two tuners.

What you need is Sky+, which has two receivers.

Od two STB's? which is messey.

To be honest, now that I've looked at what's available on Freeview (crap
mainly) I think I'll sell the STB again and go back to analogue. I reckon if
I wait a few months I'll be able to buy a reasonably priced twin tuner STB.

I notice Nokia make one but it's currently £250+. I'm sure others will
follow.