MSG2 HD was added last week, and when an overflow game is not shown, it just shows the same thing as MSG HD. Unfortunately, Verizon did not add MSG+2 HD at the same time, but word is that it's on the way. Of course, no one at Verizon will say when. When MSG+2 HD is added, it will most likely be the same thing as MSG+ HD when there's not a different live event scheduled. I hope this makes sense. Verizon is very particular about passing on the signals from the providers without altering them, so they simply will not use one channel to switch between two overflow feeds.
In my region, MASN2 HD just shows ESPNEWS HD when there's not a live event, and CSN+ HD just shows a "Comcast SportsNet" logo floating with stars moving around it. It's got to be frustrating to see two Rangers games, but hopefully you'll get all the Islanders & Devils games in HD soon.