This may be something that everyone else in the world but me already knew, but I just recently saw on an episode of Uncommon Threads this life-changing tip - In order to work with two strands together, you do not, as I previously believed, need to have two balls of yarn. You can simply use the center strand and the outer strand together and work from the one ball.
Seriously, there was a huge light bulb that came on over my head when I saw this done.
I definitely just got the light bulb too. I just assumed I'd need two balls and I was saddened by that because I have a couple of big 12 oz skeins that I was considering for some patterns but they were those 2 strand patterns and I only have one skein.