I don't personally do it, but I have a buddy that is on the SRT team here and he was showing me what they do, and it could work on semi autos too I guess.
With the departments MP5, he'd shoot three with one pull of the trigger, count it as one... that way you only have to count to 10. Ten 3 round strings, and then reload. From what he says, it's not so much for when you are out, but to know where you're at if you need to reload before you are completely empty.
I think the same could apply to your AR or FS2K. I mean, if you're ever in a situation where you are going to have to shoot someone, are you just gonna shoot 'em once? I'd think you could apply the same thing with double taps, you'd only have to count to 15.
Personally, I really can't see how it's practical, I mean if you're in a fight, you probably have many more important things to worry about than round count. Just reload when your empty, if you think you're getting close, reload anyways and retain the partial. I think you'd have to practice A LOT under high stress to be able to do all the things you'd need to do AND keep count.