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Hopefully your TV has two scart sockets? If not you might want to consider a scart multiadaptor.
The best way to wire (using scart leads) is:
STB #1 out > TV in #1
STB #2 out > DVD in #1
DVD out #2 > TV in #2
when recording make sure your DVD is recording from the DVD in #1 scart socket, not the TV connected one.
If you don't have two TV Scarts then using a multiadaptor:
STB #1 > multiadaptor in #1
STB #2 > DVD in #1
DVD out #2 > Multiadaptor in #2
Multiadaptor out #3 > TV in #1