Channel 3 in Pensacola, FL did the exact same thing a few years ago, and I never quite understood it either. Once, however, it came in handy when there were dropouts on the main channel, and I was able to flip over to 3.2, which, for whatever reason, was not having the issue. Also, it seems like I recall the picture would be "full screen" on 3.2 whenever 3.1 was showing "wide-screen" programming. Overall, though, I really didn't see the point. They later dropped it, only to return (a year or so?) later with The Country Network on 3.2.
Doesn't adding a secondary channel of any kind prevent you from showing "Full HD?" Don't you need the full spectrum to have true "Full HD?" Unless a station is getting ready to add another channel, I can’t understand the point of tying up their bandwidth just to show a copy of the main channel, although I’m sure there’s a reason.