I assume you're referring to the old FFXIII-event gear - alas those were added way before glamour was actually added to the game and back when single-handed weapons were still a viable option for CNJ/WHM and THM/BLM players - the FFXIII event first ran back in 2013 (the clue is the FFXIII weapons are actually level 13 gear and not level 1 like glam gear usually is, and has probably the best stats for any weapon at that time in the game, where as glamour gear has not stats at all usually).

Unfortunately it doesn't seem SE are that interested in updating the FFXIII mage weapons to be double handed, as that would require them having to model them from scratch (you can't just hold the single handed weapon as a double handed one, they're too small and the wrong shape). There is no actual easy fix to this sadly.