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My recommendation is for you to spend a little money and get a two into one scart adaptor, available at most decent high street stores. Some more sophisticated versions have auto switching between input a & B, others are manually switched. However autoswitching relies on the Pin 8 of the scart lead being activated by the VCR or DVD.
On the assumption that your TV has two scart sockets, wiring would be:
STB Scart #1 > TV Scart #1
STB Scart #2 > VCR Scart input
VCR Scart output > Scart 2into1 Adaptor In#1
DVD scart > Scart 2into1 Adaptor in#2
Scart 2into1 Adaptor out > TV Scart #2