Twinborn only began to appear after the Final Ascension, due to the intermingling of the Terris people with the other citizens of the Final Empire. This mingling also had the effect of diluting Feruchemical power, creating Ferrings. The interaction of Feruchemy and Allomancy in a single person allows for practical benefits of the power (Bloodmakers healing in a time bubble, Coinshots augmenting their weight with Feruchemical iron), etc.
It should be noted however, that it is difficult to be born a Mistborn and full Feruchemist. This is due to fact that they share portions of the individual's "spiritual DNA"., thus preventing a full manifestation of both abilities. Whether this means someone can be born a full Mistborn and Ferring or full Feruchemist and Misting is yet to be determined.
If the Twinborn is fortunate enough to obtain the Allomantic and Feruchemical abilities of the same metal, they can practice Compounding. Initially seen in The Final Empire, the Lord Ruler is able to compound all the metals (as a result of his Terris birth and later giving himself Allomancy). After the Lord Ruler's death, Sazed theorizes about his ability to use Allomancy and Feruchemy together which therefore granted him the ability to rapidly heal, maintain a young age and be amazingly strong. Compounding was a one-time occurrence during the time of the Lord Ruler due to the fact that Feruchemists and Allomancers did not commonly mix sexually (a strict breeding program was enforced on the Terris people) and therefore could not genetically mix the powers. Compounding is seen throughout Alloy of Law in the case of Miles Dagouter and his extraordinary ability to heal by compounding gold due to being born a Bloodmaker and Augur.
Compounding involves storing a Feruchemical attribute in a metalmind, then burning the metalmind Allomantically. This in effect creates a "new metal". Brandon explained, "In compounding, when you have the power of both Allomancy and Feruchemy, you draw power from the other place [Preservation] through the metal and it recognizes the power that is already stored--"Oh, this is healing, I know how to do that”--and so you get the power of Feruchemy but boosted by energy from the other place. This is how the Lord Ruler achieved immortality."  The Lord Ruler Compounded Atium, which stores Youth, in order to stay alive for hundreds of years. Similarly, Miles 'Hundred-Lives' Dagouter Compounded gold (Health), getting near infinite amounts of Healing.
Just as individuals with only one Allomantic or Feruchemical power have a special name (i.e. Coinshot, Archivist), each combination of Twinborn has one as well. At this time, only the combination of Crasher (Coinshot and Skimmer) is known in the books. However, Wayne claims to know the names for each of them. Excluding the God Metals (which are the stuff of legends by the time of Alloy of Law) this makes for a total of 256 possible combinations.
|The Metallic Arts|
|Arts||Allomancy · Feruchemy · Hemalurgy|
|Concepts||Metalborn · Snapping · Savants · Metalminds · Twinborn · Compounding · Blessings|
|Constructs||Steel Inquisitors · Koloss · Kandra|
|Base Metals||Iron · Steel · Tin · Pewter · Zinc · Brass · Copper · Bronze · Cadmium · Bendalloy · Gold · Electrum · Chromium · Nicrosil · Aluminum · Duralumin|
|God Metals||Atium (Malatium) · Lerasium · Ettmetal · Trellium|
|Era 1 books||Mistborn: The Final Empire · The Well of Ascension · The Hero of Ages · (Mistborn: Secret History · The Eleventh Metal)|
|Era 2 books||The Alloy of Law · Shadows of Self · The Bands of Mourning · The Lost Metal · (Allomancer Jak and the Pits of Eltania)|
|Shards||Preservation · Ruin · Harmony|
|History||Classical Scadrial · Final Empire · Catacendre · Elendel · Southern Scadrian|
|Magic||Metallic Arts (Allomancy, Feruchemy, Hemalurgy) · Metalborn · Mistborn · Misting · Savant · Ferring · Twinborn · Compounding · Atium · Lerasium · Ettmetal|
|Other Media||Mistborn Adventure Game · Mistborn: Birthright · Mistborn film · Mistborn: House War|