Pinza,
So if understand you correctly (might be missing something), you have an SW-21 with a single tuner, and you buy a dual tuner. Instead of upgrading ($$$!) to an SW-44, you can put that Separator on the back, and it will work as well as an SW-44?
Isn't it working pretty much like a splitter?
Without a power insertor, how is the signal in terms of HD quality?
How much do these normally cost?
I've had customers with this type of issue, and we always tell them they need to run the second line and the SW-44.