When knitting garments with a circular needle, it can be useful to use two or more. I’ve already mentioned using three short double-pointed needles for socks and sleeves.
Here’s another example:
I’ve been knitting a pair of baby overalls. The legs are knit separately first (using the set of three short needles) and are then joined together with some added stitches in between. Because of the nature of the garment, it becomes very difficult to knit in a circle immediately. Therefore I use two circular needles the same size and length and have half of the garment on each needle. You can either use a third needle to knit onto, or just use the other end of the same needle.
This same technique can be used when joining the sleeves and the body of a pullover or cardigan, when making raglan decreases or yoke decreases. It is, of course, sensible to get everything onto one needle after about 5 cm of knitting, but you’ll find out yourself when enough length has been knit to allow you to easily put all on one needle. On a pullover I would have each needle go from the middle of one arm to the middle of the other arm. On a cardigan it works best with three needles, with one front and half of one sleeve on each of two needles and a long needle with the back plus two halves of the sleeves.
Happy knitting!