Kensington Trackball Works, how to scroll horizontally with Trackball or Auto Scroll
The old driver used to have an option to set a button (or button combination) to 'change scroll direction'.  I used to use that all the time and they have since gotten rid of it.

The workaround I've been able to come up with is to program a button to simply type the shift key (I use the two bottom button combination).  It's not as nice, but then you can hold the button(s) and scroll in the opposite direction without using your second hand.