Ooh, so like,
It kinda seems like both pairs of saintess/princess are needed to tackle the crisis on multiple fronts
If they work together, maybe even from different ends, they can eat away at the abnormally dense miasma until it's gone
Further proven by how both saintesses look like they're meant to compensate for the weaknesses of their respective princess and enhance their strengths.
(We alr see this with the main pair, but for the other pair, maybe its something like Tsubasa[?] helping her grow in the personality and worldview department...)