One wire is the supply which is always live - this one is connected to COM (Common). The other wire is the wire going to the light fitting - it can be connected to either 1 or 2 - doesn't matter - this one is only live when the switch is in the on position.
You must have a 2-way switch otherwise you wouldn't have 1 and 2 (or L1 and L2).
The supply should be the red wire however you can't just assume that. The safest way to know properly is to use a non-contact voltage detector (e.g.Fluke voltage detector) to check which one is which (the detector will light up when close to the live).
If you connect the red to com and the black to 1 or vice versa (or the red to com and black to 2 or vice versa) - it will still work normally - however the problem will arise if anyone tries to add extra lights in future.
If you think that's confusing then the safest thing is to get the person who won't be confused - i.e. a qualified electrician!
J, October 2008
