Two decks spread across nine tableau piles and two passes through the stock keep this Klondike cousin simple, so you can chase an inviting 86% win rate.

Albert Morehead and Geoffrey Mott-Smith built Gargantua Solitaire as a two-deck version of Klondike, spreading the deal across nine tableau piles instead of the usual seven and giving you two passes through the stock, one card at a time. The game is simple, with an 86% chance of winning, and it takes a mix of skill and luck to bring one home.
Gargantua belongs to a whole cluster of two-deck Klondike variants on the site. Double Klondike Solitaire and Pantagruel Solitaire share its nine-pile layout with small tweaks to the stock and fill rules, and Batsford, Harp, Steps, and Moving Left are all built the same way. If you'd rather start with the original, Solitaire is the one-deck classic this whole family grew from.
If you run into anything odd or have an idea that would make the game better, please contact me.
Enjoy playing!
8 foundation piles: Build up from Ace to King in the same suit.
9 tableau piles: Built down by alternating color. A run of cards in alternating colors can be moved together as a group. Only a King, or a valid run led by a King, can fill an empty space. The piles are dealt in a triangle: one card to the first pile, two to the second, three to the third, and so on, with the top card of each turned face up.
Stock: Click to turn over 1 card at a time to the waste. You get two passes through the stock.
Waste: The top card can be played to the foundations or tableau.
There are eight foundation piles.
A card can be added to a foundation pile only if it's one rank higher and the same suit as the pile's current top card, so the only card that fits on a Q of diamonds is a K of diamonds. There can be no more than 13 cards in a pile.
The top card of each foundation can be moved back into play if another pile will accept it, though on the tableau that generally means a King moving into an empty pile, since only Kings can start a new one there.
There are nine tableau piles, with one card in the first pile, two cards in the second, three cards in the third, and so on. The top card in each pile is dealt face-up, while the remaining cards are dealt face-down.
A card can be added to a tableau pile only if it's one rank lower and the opposite color of the pile's current top card, so the only cards that fit on a J of diamonds are the 10 of spades and 10 of clubs.
Cards on the tableau that are not covered by another card are free to be played onto the foundation or any other tableau pile.
Only Kings may fill empty spots in the tableau.
If the cards are alternately red and black, groups of cards in succession from high to low can be shifted from one tableau column to another.
There is one waste pile and the remaining cards comprise the stock.
When you click on the stock, one card from the stock is dealt to the waste. You get two passes through the stock.
The top card of the waste can be played to the tableau or foundation.