It should be noted that the metals a Twinborn uses for Allomancy and Feruchemy do not have to be the same. If however, a Twinborn shares a single metal with both Allomancy and Feruchemy, its power is dramatically increased. For example, Miles Hundredlives from The Alloy of Law is a "Bloodmaker". Bloodmakers use gold, which Allomantically isn't very effective, while Feruchemically is for healing yourself. However, when you are able to both burn as well as store gold in a metalmind as Miles can, you can heal extremely fast. Having a shared metal also allows you to compound the Feruchemical power by burning its metalmind, releasing the charge (in this instance, healing ability) tenfold.