I think a bit of clarification may be in order: vehicular dual turrets (TO p. 347) are "[a]vailable to ground-based and naval-based Combat and Support Vehicles (but not airborne units such as VTOLs, Airships, Fixed-Wing units, and Satellites) [...]".
I'm unsure which of the two categories to sort Wing in Ground Effect vehicles into, though. On the one hand, they're clearly airborne (even if it's just a few meters off the ground) whenever going anywhere at anything like speed, on the other they're just as obviously ground-based enough to already carry regular turrets (as the Nishikigoi's turret-mounted quad Thunderbolt battery amply demonstrates). So, can they carry a second turret as well or not?