sys/dev/pckbport/synaptics.c attempts to scroll based on Synaptics' "finger width detection". It was intended to be the case in NetBSD that scrolling is activated when two fingers are held together on the pad. Synaptics basically broke this feature in their hardware at some point (my hardware's reported figures oscillate horribly). Try lowering sysctl hw.synaptics.finger_scroll-min then holding two fingers together on the pad. It might work for you!
sys/dev/pckbport/elantech.c scrolls when two fingers are detected on the pad, regardless of their proximity. This is much easier to do and much cleaner. Briefly, the synaptics driver did this, but I had to revert it because it broke other gestures that are required on trackpads that lack buttons entirely, and I don't have hardware to test those (check the CVS history). I'm happy with arrow keys and trackpoint scrolling, tbh.
Anyway, this is probably also possible to emulate using Xorg's mousedrv(4) - see the EmulateWheel option.
Some reading https://marc.info/?l=netbsd-current-users&m=158450374625332&w=2