Double Solitaire is the exception to the solo-game rule that we've come to learn. I had to include it, though, because it is how I learned to play Solitaire (not to mention the antics of my Solitaire-crazed parents).
The object is to build eight foundation piles starting with Ace and building up in ascending order to King. The game is played with two decks—by two people. You each get a deck and you each lay out your own tableau.
You lay out the cards in the standard (Russian) Solitaire way and begin to play in the exact same way as Russian Solitaire.
You deal three cards at a time from your stock pile. The top card of the cards dealt is the available card and can be played either to the foundation or to your tableau. You cannot play to your opponent's tableau. You can assist your opponent if you want. Believe it or not, if you help your opponent, you may keep the play going. He or she might think there are no more moves left in his or her hand, but if you see one, feel free to point it out in order to keep the game in motion.
You play the game as though playing Russian (standard) Solitaire—playing cards to your own tableau. Your opponent will be playing his or her tableau and you'll be playing yours. You don't have to take turns, you just play at whatever pace you want. But keep your eye on those foundation piles and make sure you're putting down at least as many cards on the foundation as your opponent. The difference between Russian Solitaire and Double Solitaire is that in Double Solitaire you can put any card on any foundation pile whether it was started by you or your opponent. This is where the fun begins!
You win the game when you're the first one to use all your cards to build the foundations. In order to win, you cannot have any cards in your stock pile or in your tableau.
In Russian and Double Solitaire, I like to play all the cards from my tableau to my foundation before dealing any cards from the remaining stock pile. I like to know that all the visible cards have been played before laying down any more cards in front of me.
In Double Solitaire, remember you're playing with two decks and two people. Be sure to have identifiable decks because you are going to mix up the cards (hopefully not by way of a tantrum or tickle fight) and you need to know which deck is which at the end of the game. If the game is a stalemate, you can always count the cards on the foundation piles to determine a winner. The person who laid the most cards on the foundations wins the game.
The rule in Solitaire regarding “group” moves is really up to you. But in Double Solitaire be sure that you and your opponent are on the same page about the rule. A “group” move means that if there's a card you need buried in the middle of the fan of your tableau, and you have a place to move the group, then you can do so to free up that buried card. For example, let's say you have a 5 of Diamonds buried in a fan on your tableau and you need that 5 to put on the 4 of Diamonds on one of the foundation piles. If you have a 5 of Hearts open and available on one of your tableau fans, you can lift the group of cards starting with the black 4, move it to the open 5 of Hearts, and you've officially freed your 5 of Diamonds to play on your foundation. My parents never agreed on the issue of “group” moves, and that was the cause of many card-flipping moments. It was quite amusing to watch, but if you want to avoid any family conflicts, make sure all players agree on that maneuver.