You can put the second wire in either L1 or L2 it does not really matter, what it will do is change which way the switch goes for on and which way for off. In domestic premises we normally switch down for on and up for off and with writing correct way up on switch this would be using L1, however if you need to fit switch upside down because of cable length, then moving wire from L1 to L2 would mean switch is still down for on.
Large switches work the other way around, any isolator down is off so something falling on a switch will always switch it off. So really does not matter if switch does work wrong way around.
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