A Mistborn burning metal and Duralumin gets a burst of power and uses all of the metal he has. Does the size of the burst change based on the amount of the other metal ingested (i.e. would swallowing a lot of steel result in a more powerful Steelpush)?
According to Brandon (and has @Radhil commented) in this WoB duralumin burns your whole reserve in an instant
Questioner What happens if you burn duralumin while Compounding?
Brandon Sanderson Duralumin while Compounding. So, what duralumin does is it burns out of all of your metal in one burst. So it doesn't necessarily gain you power, it makes it all happen at the same time. The same thing would happen.
though it may be possible that you run out of duralumin before your other stocks depending on relative quantity:
Questioner I asked the question about chromium vs a Compounder with both invested and un-Invested metals in both their stomach and piercings.
Brandon Sanderson What it boils down to is this:
1) Yes, the piercings will get burned off.
2) The non-Invested metals go before the Invested ones. He said that because invested metals are harder to [affect], it takes a little extra time and effort to get them to burn off. So a Leecher trying to clean out a Compounder would have to get a good grip and hang on for a few seconds.
3) Chromium burns about as quickly as duralumin, so if you're trying to burn off a lot of metals, it is possible to run out of chromium before your target is clean. This would probably only be an issue when dealing with larger pieces (like jewelry) rather than your standard metal-flakes-in-the-stomach deal.