The additional rules are best illustrated by example. So let's say it is your turn and the card showing is the queen of spades. If you also have the queen of spades in your hand (remember, there are two of every card), and you play it, you get to play again (actually, you must play again), but the next card you play is as if it was played by the player who would have ordinarily gone next (and the next player doesn't actually get his turn because it is as if you played for him). So, if after playing the second queen you put down the five of spades, everybody gets a card except for the guy next to you (i.e. you have just given yourself a card). Sixes, however, are an exception to this new rule; sixes in double-deck are exactly the same as they are in single-deck creights.
There are some interesting applications of this rule. Suppose you are playing in a four-player double-deck game (although why you would use two decks for only four players is beyond me). The player across from you plays the four of hearts, skipping play to you. If you also have the four of hearts, and play it, your next card is as if the player across from you had played it (not the player you skipped), so if you play another four, it's your turn again. If, however, you play a seven instead, YOU get stuck with the card. (Moral of the story: don't play that seven.)
The other major rule change is that, at your turn, if you happen to have two of any given card, you can play both of them simultaneously (assuming it is a valid play). In this case, the second card again counts as if the next player had played it. So if you happen to have both 10's of hearts, and it comes around to you in hearts, you can play both; the first reverses the direction, and the second reverses it back so that it is again your turn. You can stack double tens like this for as long as you have them. (You can also throw sixes in just to mix it up.)
The two rules can be combined; for example, if the eight of clubs is showing, and it is a five-player game, you can play the eight of clubs, call diamonds, and play both fours of diamonds, skipping it all the way back around to yourself.
One danger of this new rule is that if you play the same card as the one showing, you must play again; this makes it very easy to crank yourself. As an especially brutal example of this, I will recall the time a crank came around to me with the value already at 17 (yes, there are twice as many crank cards in the deck). The top card was the two of clubs, and the only crank card left in my hand was... the two of clubs! So I was forced to play it, to increase the value of the crank to 19, and then to play again. Since I couldn't, I got to draw the 19 cards. (It was small consolation that the next player didn't get to go because I took his turn.)
The reason double-deck creights is so bloody is because it is possible to do large amounts of damage at a time; for example, getting cranked for 19, or having somebody play triple fives. In order to cut down on the violence, as I said, one set of fives is removed; however, it is customarily shuffled back in the deck for the last round of the game. (And if you thought there were a lot of shuffle pressures in the earlier rounds, you ain't seen nothin' yet.)
That's all for the double-deck rules. If you have any questions or comments, mail
pahk@fas.harvard.edu