Most of the knitting I've done in the last little while has been with 2 strands held together. I usually have them in separate bowls on the floor and just pull them up as I need them. There's a bit of tangling but not too much with this method.
I tried winding the two balls into one but found it was just easier to do it the other way.
When I do two yarns together it is usually because the yarn I have is not the recommended weight that the pattern calls for. Putting two together often solves this problem... and, depending on what you are looking for, you can have two different colours so that they bounce off each other or two that compliment each other very well.
So far I have combined a dark yellow with a varigated green in a hat. They look so nice together.
But I have a lot of gray and black I'm knitting up to de-stash and they look really good together as well.
One of the best things, I think, about using two strands together is the surprise you get at the end of the knitting from the patterns the two have formed. So lovely...