I have a Pioneer DVD/HDD player recorder that is supposed to have the timeslip function, albeit that I haven't used it this way.
Simple to connect (in place of a VCR) via SCART leads
STB#1 > TV #1
STB#2 > DVD/HDD in
DVD/HDD out > TV#2
Only issue is the programming of the STB does not trigger the DVD/HDD in the same way as SKY+ boxes are integrated, so you need to be a bit more clever in setting up both STB and DVD/HDD to start/stop together.
Dunno about SKY, but as NTL's EPG only runs for three days that can be an issue for advance programming of many channels
