You can pull the end of yarn out of a ball, if it's wound properly, from both the center and the outside. You can probably find the end on the outside easily. Can you appreciate a sort of opening in the center of the ball? Dig into there and gently pull out the tuft. Fiddle with it until you find the other end of the yarn. Voila! You have two ends, hold them together and knit as if they were one. You are knitting double stranded.
More trouble is to make two balls out of it by unwinding half into a second ball.
"One can never have enough socks."
A. Dumbledore, Book One, Chapter 12