If you have seen the "Fancy-Pants" scarf in the shop, you'll know that I used Crochet on the Double to make it, using one skein of Prism Stuff and one skein of Merino Mia. If you use a large hook (I used a K), you can make two skeins of yarn go an awfully long way.
I'm going to put a short, quick explanation of the technique here in this blog entry.
For a tutorial that goes into much more detail, download this pdf (three pages long) for a much more in-depth explanation, with more pictures.
CROCHET ON THE DOUBLE
This technique uses two yarns, referred to here as Color A & Color B.
1. Using Color A: Make a chain, the number of stitches desired +1.
2. Using Color B and working right to left: Insert hook into one side of the chain, skipping first chain, yarnover, and draw up a loop.
3. Repeat step 2 in next and every chain across. You will have "knitted on" a row of stitches.
4. Turn your hook over
5. Using Color A and working left to right: Crochet the stitches off your hook, pulling through one loop the first time, and two loops the rest of the way across.
6. Using Color A and working right to left: Knit stitches onto hook, inserting the hook from right to left under the vertical bars created by Color B in the row before. Skip the first bar.
8. Using Color B and working from left to right: Crochet the stitches off your hook. (Remember, first stitch through one loop, the rest through two loops.)
9. Using Color B and working from left to right: Knit on. (Remember, skip the first bar.)
Repeat steps 7, 8, and 9, until your pieces is as long as you want it to be, or until you are out of yarn.
I remember the color sequence this way:
Knit on using the same color as the loop left on your hook.
Crochet off using the opposite color as the stitches loaded on your hook.