I am using BetterTouchTool to add more gestures for my Mac. I want three fingers swipes to move me back through the previous apps that I have open. Currently, I am setting the gesture to Cmd-Tab. Unfortunately this cycles me through the last two apps. I want to be able to swipe twice to go back two apps. How can I add this ability to BetterTouchTool?
Here are two options. They both solve your general question (how to use swipes to cycle through previous apps), but not your particular use case (use two consecutive swipes to get to your second-from-last used app). I've also thrown in a bonus option, which I personally use, due to the limitations of the first two options.
1) via Keyboard Maestro
In KM, open up the Macro Library (in the Window Menu). Under the "Application Control" category, you'll find "Switch to Last Application" and "Switch to Next Application". The second one will cycle you through apps in one direction: from right to left in the list that appears when you press CMD-Tab. Give this macro a hotkey trigger and tie it to three-finger swipe with BTT.
Downside: Because "Switch to Last Application" only switches between the last two active applications, and "Switch to Next Application" cycles through all applications in one direction only, you are effectively limited to one-way cycling through apps. And because it cycles from right to left in the CMD-Tab list (rather than from left to right), you won't be able to swipe twice to get to your second-from-last used app. (Inspect the order of the CMD-Tab list to see what I mean.)
2) via the Dock
In the Keyboard Preferences Pane, under the Shortcuts tab, and within the Keyboard category, enable "Move focus to the Dock". Give it a hotkey. Then use BTT to trigger this hotkey with Three Finger Swipe Left, and attach two additional actions: arrow left, return. Make another BTT trigger for Three Finger Swipe Right, and for this one have your additional actions be arrow right, return.
Downsides: This method switches between apps according to their order in the dock, rather than according to how recently you've used them. So again, you won't be able to swipe twice to get directly to your second-from-last used app. Also, this solution is not as smooth as the first. You may need to add pauses between keystrokes in BTT, especially if you keep your dock hidden.
3) Bonus option! (Because I don't think there's a way to perfectly accomplish what you're asking for -- I've tried many times.)
I like to keep Three Finger Swipe Right and Three Finger Swipe Left both tied to CMD-Tab. It's a redundant but logical way to switch back and forth between your two most recent apps. Then I tie Three Finger Tap to a Keyboard Maestro hotkey which executes the "Activate Application Switcher" action. I have it display the switcher as horizontal icons, sorted by use order, and positioned under the mouse curser. This basically puts a smaller version of the CMD-Tab list directly under my cursor. It makes it quite easy to switch between apps using three finger swipes and taps.
If you don't mind adding a modifier key to the gesture. Hold the key, say CMD, and swipe twice.