Touchpad scrolling options behave as if mutually exclusive but the corresponding checkboxes can still be selected
In Trisquel 11 Aramo, Control Center > Mouse > Touchpad tab.
In the 'Scrolling' section, it is possible to have all option checkboxes ticked: horizontal, vertical, two-finger and edge scrolling. But horizontal scrolling seems to be available only if the corresponding vertical scrolling mode is selected (two-finger vs. edge). Also, edge scrolling is not available if both scrolling modes are checked. So the actual behavior is similar to what would be expected from the first screenshot there: https://github.com/mate-desktop/mate-control-center/issues/228.
The closest discussion upstream is this one: "Touchpad settings: Support edge and two-finger scrolling independently" https://github.com/mate-desktop/mate-control-center/pull/229. Not sure whether this means that edge and two-finger scrolling are supposed to be mutually exclusive, or simply that they can be activated independently from each other. They clearly seem to cancel each other when both are checked, with two-finger scrolling having priority over edge scrolling.