The one pattern, I saw it on, was not old, it was a vest made with alpaca DK weight, and the other was in a book, which I am not sure of the age, but it looked like a light weight summer sweater, then when you read, it was made with a double yarn DK weight. Just sort of didn't make sense. I know the vest when made up, you could not tell it was made with two strands.
So, other than a certain texture or color, it sounds like I could just use a heavier weight yarn and only one strand. Thanks