Presumably the design engineers did the calculations and decided that, to give reliable performance under the most adverse conditions likely to be encountered, two belts would be necessary. Even in those pre cadcam days they wouldn’t have incurred the extra costs involved just for the fun of it!
However it’s interesting to note that a single A25 cogged belt was specified as the replacement for the twin plain A25 belt drive used on one of the early Mountfield cultivators. The Mounfield belt chart of the time notes the change just as a running substitution with no other engineering changes; so presumably the designers had either over-estimated the power requirement, or the advent of a belt with increased power handling allowed them to make a cost saving. Also, by using a single belt they avoided the “matched pair” requirement.