I have two of the same kind of drift socks that Kevin showed in his picture. I have used one/both a number of times to control my drift speed or drift angle when fishing. I've tried rigging them in parallel (one off each transom tiedown point), and in serial (one behind the other). I've found that if I run two, serial is best.
Don't confuse drift socks with sea anchors.
Drift socks are smaller--typically 24"-48" in diameter and used to slow your drift or turn the boat so that it drifts bow first, transom first, or gunwhale first.
Sea anchors are big--typically 12' in diameter or larger and are normally used in high winds and high seas when you've lost power and need to keep the bow into the wind for safety. While drift socks are for convenience, sea anchors are for safety.