The other way to do it is look at the metric size of the needle recommended for your yarn: size 8 needles are 5mm. Double the 5mm, which gives you 10mm, which is a size 13 needle. That is the size you would need to double strand that yarn.
You can work backwards too. I had a sweater to knit that required three strands knit on size 19 needles. I wanted to use a yarn that was a little thicker, so I thought I’d double it instead of triple. But I wanted to make sure that it would turn out. So I figured out what size needle I would need to double strand my new yarn choice and it was a 19. Perfect match! Of course I’ll still have to do a swatch. I hate swatches.