This one is fairly well answered, but I may have a bit of insight into why you see twin-linked being used multiple ways. There are, as far as I can tell, three possibilities, in increasing order of probability.
1) People are being dirty cheaters.
2) People don't understand the rules.
3) People are rolling both to-hit dice at the same time. As long as one scores a hit, then the gun has scored a hit. This is done instead of rolling one die at a time, as it speeds things up. You would not, of course, hit twice if both scored a hit. My friends and I tend to roll this way frequently.