Solitaire is typically a card game played solo, but it can be played with two people. This type of solitaire is called "Double Solitaire", according to Pagat. Each player must have a 52-card deck. Pagat's website suggests that each player use decks whose backs are colored differently to help score the game when it is finished.
Begin by shuffling each deck of cards. Players can shuffle the opponent's cards to avoid one player "stacking" the deck, but this is not required. Lay out seven piles. Players familiar with traditional solitaire understand how this works: the first pile contains one card, face up; the second pile contains two cards: the first is face down and the last is face up. Continue until you have seven piles. The last pile should contain six cards face down and one card face up.
Reserve Space for Eight Foundation Piles
In the center of the playing area, reserve space for eight foundation piles. Foundation piles begin with an "Ace" card, placed face up. Since two decks are in play, there are eight aces available, hence eight spots for the foundation piles. During play, either player may play cards into the foundation piles (by placing an ace into an empty slot or placing the next higher number of the suited ace in a pile). For example, assume player 1 placed the Ace of Hearts into the foundation. Player 2 has the Two of Hearts face up in one of his piles; Player 2 can place the Two of Hearts onto the Ace of Hearts in the foundation pile even though Player 1 laid the Ace of Hearts there.
Use Traditional Solitaire Rules; Alternate Turns
The card in the first pile with the lowest number determines who goes first. Therefore, if player 2 dealt a three of diamonds as the first card when making the original seven piles, and player 1 dealt an eight of spades, player 2 would go first. Double solitaire plays according to basic solitaire rules: players can deal three cards from the remaining deck of cards but can only use the top card from this three-card draw. Players can move cards that are face up onto other piles or into the foundation according to these rules: the card must be one degree lower and must be of the opposite color (so the seven of hearts may be placed on below the eight of spades or the eight of clubs). When the first player runs out of moves (in that he must turn over more cards from his or her deck), that player's turn ends and the other player's begins. Alternate turns.
Player with Most Foundation Cards Played Wins
When both players run out of moves, the game ends. To determine a winner, count the number of cards each player played into the foundation piles (this is why it helps to have different colored decks). The player with the most played-foundation cards wins.