In the UK Usually - Power goes to the switch on one colour (RED) and power comes out of the switch on a different coloured wire (BLACK) so take care. (New wiring colours have Red as Brown and Black as Blue.) The switched live wire is not what its colour suggests and has to have an identifying sleeve on it to show that it's dangerous.
The live return from the switch (switched live, line, phase - the dangerous one) needs to be looped to BOTH sets of spots - each set of spots also needs a neutral return wire to complete the circuit. (You may well have used the dangerous wire as if it were a neutral on the non-working set of spots and consequently have live on each side of those lamps)
If this is all too much for you better get an electrician in.
Peccavi
November 2008
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