Right now in the game on it's current state, you need more "affinity xp" and more beakers to reach purity 15 than you need to reach purity 8/harmony 7, because earlier techs are cheaper and late affinity quests are harder to fulfill.
So they can't make it as simple as "your level of purity/harmony is the addition of your lvls in both affinities", that would put pure affinity civs in disadvantage.
But they also can't make your hybrid lvl be the lowest denominator, like, "purity 10 harmony 7, so your hybrid lvl is 7", that would put the hybrid player at a huge disadvantage.
What i really would love to see is affinity being disconnected from tech tree. Or at least from specific techs. It would be nice to research an leaf tech, and get an option on which affinity to pursue. For example, researching alien genetics, which would pop up a quest window with 3 choices: use alien dna to better understand this world (unlocks an harmony building and 1 lvl of harmony); study alien genetics to adapt plants and stuff from this world to our needs (purity building and +1 lvl purity); study alien dna to whatever-whatever (supremacy building and lvl). And if you have enough lvls in 2 affinities, it would give you some extra options to advance in that specific hybrid combination.
But this would still give pure affinity players an edge in upgrades...
So i don't really know
I hope hybrid affinities have their own unique unit looks, because just being able to train both cndr and xeno cavalry is too lame, those two units really don't look like they belong to the same nation.
So they can't make it as simple as "your level of purity/harmony is the addition of your lvls in both affinities", that would put pure affinity civs in disadvantage.
But they also can't make your hybrid lvl be the lowest denominator, like, "purity 10 harmony 7, so your hybrid lvl is 7", that would put the hybrid player at a huge disadvantage.
What i really would love to see is affinity being disconnected from tech tree. Or at least from specific techs. It would be nice to research an leaf tech, and get an option on which affinity to pursue. For example, researching alien genetics, which would pop up a quest window with 3 choices: use alien dna to better understand this world (unlocks an harmony building and 1 lvl of harmony); study alien genetics to adapt plants and stuff from this world to our needs (purity building and +1 lvl purity); study alien dna to whatever-whatever (supremacy building and lvl). And if you have enough lvls in 2 affinities, it would give you some extra options to advance in that specific hybrid combination.
But this would still give pure affinity players an edge in upgrades...
So i don't really know

I hope hybrid affinities have their own unique unit looks, because just being able to train both cndr and xeno cavalry is too lame, those two units really don't look like they belong to the same nation.