Not sure if this has been discussed here or not, but I think the “Two-Toa” combiners are designed to be the upgraded versions of Tahu and Kopaka to combat with their respective elemental beasts.
First, their choices of which Toa to combine together seem to get along with what some considered to be the combination of element for the beasts.
Lava beast = fire + stone = Tahu + Pohatu
Storm beast = ice + water = Kopaka + Gali
(Quake beast = earth + jungle = Onua + Lewa ?)
Second, these combiners were designed to have Tahu and Kopaka as the prominent features with Pohatu and Gali being only additional parts and armors, making them look more like upgraded $20 Toa rather than actual fusions of the two. And guess whose corrupted masks the beasts come with?
Finally, this might be a bit of a strech, but since it is now confirmed that Lava beast could fly, guess which of the Two-Toa have the wing-like add-on. Yep, it’s Tahu combiner, the character shown to battle the Lava Beast.
I’m not saying these combiners are gonna be canon upgrade storywise, but I just think they were designed for kids to rebuild their Toa to battle their respective Beasts, encouraging them to play with last wave’s Toa when the Beasts are out. If this is true, we might even get manual for the Onua+Lewa combiner later this year. I mean, Quake Beast need to fight with someone, right?
EDIT: I also have a thought about the state of Okoto before and after the Great Cataclysm I (kinda necro-) posted in 2015 topic and didn’t received many responds, which this topic might be more suitable since the theory involve the Creatures which appeared the year. So I’ll post it here again.