There are dozens of games which use two decks of cards, and many are available on the Internet or through computer programs. However, it's often more satisfying to play with real cards. The most popular solitaire game, Klondike, can be played with two decks.
Shuffle both decks of cards together. Form nine tableau rows. Turn one card face up on your left and deal eight cards face down to the right. From right to left, deal all cards face down until you reach row 2. Deal a card face up on rows 2 and 3, then six cards face down from left to right, then five cards right to left.
Deal a card face up on rows 4 and 5, four cards face down left to right and three cards face down right to left. Deal a card face up on rows 6 and 7, then two cards face up from right to left and one card on row 9. Deal a card face up on rows 8 and 9.
Set the remaining cards face down to the side. This is called the stockpile. Play solitaire as usual, in descending order and alternating red and black cards. When you find an ace, create a foundation row and begin playing cards in ascending order in the same suit as the ace.
Deal three cards from the stockpile when you run out of moves. Turn over the third card and see if you can play it. You can cycle through the stockpile as many times as you can in search of a card that will allow you to continue playing.
Win the game by placing all cards on the foundation rows.
Turn over a face down card at every opportunity. To win the game, you must uncover all face-down cards.
Don't be too quick to play cards on a foundation row. It's often wiser to keep the card in play.