Allomantic and Feruchemic Combinations.
If an Allomancer Mistborn also can use Feruchemy, then certain synergy combinations can result. This is most evident in the Lord Ruler.
If a Feruchemist taps an ironmind and steelpulls or ironpushes, then they can increase their weight to become the anchor for the pull, even if an enemy also manipulating the piece of metal weighs more, or the metal weighs more than the user. One could also decrease their weight to gain more movement of your body from each push or pull.
A combination of Feruchemy and Allomancy allows the user to somehow gain immortality. This is known to use Atiumminds, and Goldminds with ability to burn Gold as being a Augor or a mistborn, and possibly burning pewter.
If a Feruchemist taps a Pewtermind, Steelmind, Bronzemind, and Goldmind and Allomantically burns Pewter, they will gain increase in Strength, Speed, Wakefulness and Health beyond what one would normally be able to achieve. This will work if you tap smaller combinations of those minds, however, only for the attributes you tap.
Likewise, if a Feruchemist taps a Tinmind and Allomantically burns Tin, then their senses will increase beyond what one would normally be able to achieve.
If one Allomantically burns a metal then fills the Metalmind of the same attribute, they can fill the metalmind without lessening that attribute. This is why one might burn pewter to fill a Goldmind, then fill an Atiummind and tap the Goldmind, possibly creating the Lord Ruler's ability to live forever.
By burning a metal containing a stored attribute, such as burning atium which contains youth, the user effectively makes a profit on the attribute stored in the metal due to allomancy's property of drawing power from the metal. Thus the user gains more of the attribute than invested. An example of this is the Lord Ruler's immortality which is achieved through periods spent aged, storing youth in atium which he later burns and stores in the main atium minds which sustain him. Note that the attribute stored must have orignated from the user, hence Vin's inability to use the power in Sazed's pewtermind.
The term for a Twinborn who can burn the same metal as their Feruchemical ability uses is called a Compounder. A Compounder can store that metal, and tap it later at tenfold the power of a normal Misting.