keyboardmouseterminaltrackpadvi
I am using a dell mouse with my mac. Although in most context scrolling the mouse wheel produces the same effect as swiping down with two fingers, it sometimes happen that the mouse does not work. One example is with vim (terminal).
I press the shift key and scroll: nothing happens. I press the shift key and swipe down and the page shifts.
EDIT
I tried with Better touch tool but it is of no help.
Normally, this means you just don't put both fingers at the same time. Try sticking your index and the other finger together and moving your arm or your wrist instead of trying to move the individual fingers. You'll get used to it.
The animated videos in the System Preferences -> Trackpad are a good example of the safest way to do the gesture. Once you get the hang of them, do them like you want, but it's a good starting point.
Update 2020/12/03: As other's have pointed out, this is not a feature of Karabiner in newer versions.
It appears that this is integrated in the Karabiner app so it works out of the box without any custom XML (which are described above).
You'll need the following settings:
Set the Enable Natural Scrolling setting in the Mouse/TrackPad PrefPane to 'enabled'. This allows for the internal trackpad to work as it's supposed to. Any connected non-Apple external mouse will scroll in their own natural way.
Best Answer
Shift/scrollwheel is horizontal scroll, which will only work, of course, if there is somewhere to scroll to horizontally.
I've never used a trackpad, but I presume horizontal scroll on a trackpad can be quite specifically indicated by horizontal movement - it can't, of course, on a mouse-wheel.
As noted in comments by timothymh, SteerMouse ($20) can override this behaviour & set Page Up/Page Down to the shift/scroll action.