I believe what is meant by a dual box is actually what is called as an analog pass through box.
This type of box will allow you to watch digital signals, but pass through an analog signal. In the case of WMUR, if you are in the northern half of the State using one of their translator stations, they have an extended period before they will convert to digital. Thay have been advertising this for some time on thier main station.
Not all stations will go digital on the date advertised, including some in this State.
For a little better explanation, check out
This informative link.