Just rung the Gold line and they say they can't upgrade one person and leave the other on the same booking. They didn't offer to attempt to split the booking or anything like that.
I asked about OnBusiness but they don't know how OB works. But that suggests I am going to be unable to upgrade one of the seats using my GUF2 and then pay to upgrade the other one with OB points.
That's a touch annoying.