Hi guys! I am new here and I had a thought about the twinborn. Sorry if this has been posted before. (Excuse me for any mistakes, english is not my native language)
So. We know that a twinborn has access to one allomantic and one feruchemical power. This is possible because of the fact that they had both allomancer and feruchemist ancestors/parents. We also know that by Wax's time, there are no Mistborn, only Mistings, and no full Feruchemists, only Ferrings, as the wiki said that that was the cost of twinborn becoming possible. The powers intermingled with each other, creating the twinborn.
We also know that there were mistings waaaay before Sazed's Ascension, but no ferrings. Why? Feruchemy was much older, it should have been the first to start becoming weaker (though that might be because the remaining feruchemists after TLR's Ascension tried keeping the bloodline pure, but let's ignore that for now). Mistings and mistborn also had to snap to access their powers.
-Feruchemy has a tendency of being whole and not having to snap
-Allomancy has a tendency of having to snap and only giving access to mostly one power
What does this mean? As the sDNA of the two metallic arts mixed they created something new. They kept some traits and threw away the rest. Feruchemy took on the tendency to only have one power. And Allomancy here it comes, be ready, took on the tendency of not having to snap.
Why I think so? We have been told before that after Sazed's Ascension, the method of snapping was changed. I also don't remember Wax or Wayne saying anything snapping-related (correct me if I'm wrong) in all of alloy era. It was also said that metalborn are much more common in Wax's time, which kinda feels connected to my theory. At the age of, let's say 10, each child is given a vial of all the sixteen (fourteen, as you are pretty useless as an aluminum or duraluminum gnat unless you are a compunder) metals to drink, thus finding the allomancers. The main problem in the Final Empire was that even if someone snapped, they would have no idea they are allomancers (except if you snap for a metal you can get from food or water, like Vin did). In the Alloy Era, such a problem doesn't exist anymore.
These are my thoughts. Feel free to correct me if I'm wrong.