The Semispatha and the Saex

[Thomas V.]

My guess would be that Vegetius knew that the legionary of old carried two edged weapons, one larger [i.e. a gladius?] than the other [i.e. a pugio?], and that,...

...in his own time, the Roman armoury included two swords, the spatha and the semispathium, again of differing sizes. [...] His idea was to improve the army by returning to the methods of the
past but he saw no reason why it should not have up-to-date equipment. So he gave his old-style legionaries modern weapons.

If the soldiers of the time of Vegetius would have been equipped with a sword and a shorter sword(like object), there would have been no need to return to the methods of the past (i.e. "two edged weapons, one larger than the other").