The description for this achievement is a little bit misleading. It states “Hatched a Dual-Element monster”, implying a singular hatch will do, but for this achievement, you’ll have to hatch at least 80 dual-element monsters.
Each Monster has its own inherent element. Rathalos is Fire, Zinogre is Thunder, Mizutsune is Water, etc., etc. And, when found in their natural habitat with an untouched or unaltered Ecosystem, they’ll be just that – single-element Monsters.
But, through Habitat Restoration, we can introduce Monsters that aren’t native to an Ecosystem and alter the elemental properties of the Monsties we hatch from that area.
This is influenced by the elemental properties of the area, which seem to be set, and range across the 5 main Elements in the Monster Hunter Series (Fire, Thunder, Water, Ice, and Dragon) as well as Non-Elemental. There's no coverage for Status Effects (Poison, Paralysis, Sleep, and Blast).
Head to the Habitat Restoration section of your camp menu to see which areas have which element. Under my Sunpetal Plains banner in the top right, as well as the icon to the left of the area tab, it tells me the Area element is Fire.
This means that the Monsties in the Sunpetal Plains area will hatch with an additional Fire element. The inherent Fire-type Monsties become double Fire type, and non-Fire Monsties will hatch with a red recolour to indicate their new additional typing.
I tested this with a batch of 8 non-Fire Monsties, including Velocidrome, Gypceros, and Royal Ludroth – all 8 hatched red, so it’s not a chance of a dual typing, it seems to be very much a guarantee.
Once you’ve hatched 80 dual-type Monsties, the achievement is yours!
Like many of these achievements, you can check your progress in your Ranger Records by pausing the game, navigating to “Ranger Files” (one tab to the right), and then selecting the Ranger Records dossier (the red one on the right). The Bicolor Gem medal is the fourth medal on the fifth row.