This is a feature request. SUMMARY Currently those two options use radio (exclusive checkbox) and only and exactly one of those can be selected. With synaptics driver settings it's possible to have both of those options. I've also noticed that the option to disable scrolling altogether is missing. Both could be implemented by replacing radio with normal checkbox.
This appears to be a limitation of libinput itself. Only one scroll method can be active at a time, see https://wayland.freedesktop.org/libinput/doc/latest/scrolling.html I understand that this is a frustrating answer, but there's nothing Plasma can do about that.
No problem, your answer was helpful. I'll try requesting this feature in libinput then.