The guy that created the dual scale Brix/SG refractometer should be kicked in the nuts.
Since I use one of those, I'm wondering what's the problem with them?
I know your question wasn't directed at me, but I have tried two refractometers that were all over the place when compared to a hydrometer. Even with correction spreadsheets and what not, they just weren't reliably accurate for me.
I'm still mapping out the magical correction factor for my refractometer. Terrill says his factor is 1.04. Mine, so far I am calculating 0.94-0.98, but I still have a lot more data to gather to know for sure. The fact of the matter is, only a hydrometer knows for sure. The refractometer is quite swaggy and only gives ballpark figures.
And as far as a dual scale?! Ha! Virtually guaranteed to be pretty far off.
Of course, for those of us only interested in getting numbers within plus/minus 0.005 or so, a refractometer might be considered "good enough". Many times, I am one of those people. Other times, I'd prefer to be a little more accurate, so then I have to pull out the trusty hydrometer.