This is a competitive version of Klondike patience (solitaire). Each of the two players plays with their own 52-card pack. The backs of the cards should be different so that they can be sorted out after the game.
Each player deals a layout as for Klondike: 28 cards in 7 piles, each having the top card face up and the rest face down. The left hand pile has just one card, the second two, and so on, the right hand pile having seven cards. Between the two rows of 7 piles is space for eight foundation piles, which can be played on by either player. Each player begins with a stack of 24 face-down cards.
There are two ways this game can be played: the players may take turns, or it can be played as a race, both playing simultaneously.
The player with the lower card on their one-pile starts - or in case of a tie whoever has the lower card on their two-pile and so on.
At your turn you can make a series of moves as in Klondike. You may move cards around on your own layout, move cards onto the foundation piles when they fit, and move cards from your discard pile to your layout or the foundations. You are not allowed to play onto or use your opponent's layout or discard pile. You end your turn when you cannot or choose not to make any other moves by turning over the top card of your face-down stack, and placing it on your face-up discard pile. Your opponent then plays.
The game ends when one player has played all the cards to the foundation piles and wins, or when both players are blocked, in which case the winner is the player who has played more cards to the foundation piles.
The moves in this version are the same as above, but the players do not take turns. Both play at once, as fast as possible. When you cannot or do not wish to make further moves with your face up cards, you turn over the top three cards of your stack, put them face up on your discard pile and continue playing. If there are no more cards in your stack, you may turn your whole discard pile over to make a new stack, and turn over its top three cards to start a new discard pile.
As in the turn-based game, play continues until one player has played all the cards to the foundation piles and wins, or until both players are blocked, in which case the winner is the player who has played more cards to the foundation piles.
Some play that if both players are blocked and have fewer than 7 cards in their stacks, they may agree each to turn all the cards of their stacks face up in a row and use any of the cards from their own stacks to make any further moves that are possible.
A free Double Solitaire computer game playable across the web is available from Robert Nichols' solitaire.com web site.
At Solitaire Online you can play Double Solitaire with simultaneous play against a computer opponent.