Double Klondike is played the same as standard Klondike, only with two decks instead of one and takes a lot longer to play. As with most Solitaire City games, your game is timed and the faster you win, the higher your score should be. Here's how the scoring system works:

Good Points

Turning over a face down tableau card+50 Points
(max = 1800)
Playing a card from stock to tableau+25 Points
(max = 1475)
Playing a card from the tableau to the foundations+50 Points
(max = 5200)
Playing a card from the stock to the foundations+75 Points

Bad Points

Every second of elapsed time-1 Point 
Every time Undo is used-25 Points
Moving a foundation card back to the tableau-75 Points
Each re-deal after the fifth pass through the stock (Deal 3)-100 Points
Each re-deal after the second pass through the stock (Deal 1)-100 Points

Completion Bonus

When the game is completed, you will receive a bonus calculated in the following way:

Bonus = End of Game Score - (10 x Time Taken)

The maximum end of game score, ignoring time taken is 1800 + 1475 + 5200 = 8475. Therefore the theoretical maximum score possible is 16,950. However, an average game can take around 8 minutes, in which case your end of game score would be 8475 - (8 x 60) = 7995. Your completion bonus would then be 7995 - (10 x 8 x 60) = 3195 giving a grand total of 11,190