The presence of 3 wires suggests there is another switch somewhere which controls the same lights, in which case linking the 2 COM terminals won't work.
Alternatively one of the wires may not be doing anything, in which case linking the 2 COM terminals may work.
In either case you would still need to run a neutral to the new light, then a cable back down to the switch as if there wasn't already a switch there - just because there is a 2 gang switch on the wall doesn't mean you're already half way there.
Adam
October 2009