Forty-Nine
2 Decks. Very Hard (<1%). Skill/Luck balanced.
You need all your wits about you to win Forty-Nine. Worrying back essential and assembling the longest possible sequences built down by alternate colours.
When you manage to get two empty columns there is a clever strategy you can use. For example, lets assume you have a sequence of cards from a black jack downwards on top of other cards in a tableau column. In another tableau column you have a red queen that your jack's sequence could go onto. The problem is that you are only allowed to build down one card at a time. Entire legal sequences can be moved into empty column spaces however. First, move your Jack's sequence into an empty column. Then leave the Jack where it is and move the balance of it's cards into the second empty column. That liberates the Jack so it can now go onto the vacant Queen.. Repeat this process until you build all of the original Jack's sequence down onto the vacant queen one card at a time.
Another little trick involves worrying back from the foundations . Lets say you have a black Queen occupying a column all on its own. Take a red King off of a foundation and put the black Queen onto it. That means that when the King's twin brother appears it will now have somewhere it can go.