1. How do you simulate the effect of burning both metals at the same time (Rioting a desirable emotion and Soothing everything else.)? I was thinking that you could pick which metal you wanted to use and then add either 1 or half your rating with the other metal.
Koel (B 4, Z 5) decides to use both metals in an attempt to bewitch a useful informant. She rolls either Zinc 6 (5+1) or Zinc 7(5 + (4/2))
I would have the character make separate Challenge rolls for soothing and rioting, as the Difficulty of the Challenges might not be the same number. This would also help in descriptions of how the character is influencing their target. Remember, the Outcome of the soothing and rioting generates bonus dice for Charm and Influence rolls against the target. If a character succeeds at a soothing roll, but fails at a rioting roll, the character still gets a bonus (albeit, a smaller one) to Charm and Influence rolls.
2. It seems to me that Savants should get a bonus of 2 on Tug or Push Emotions. Also, if a Mistborn has both those Stunts, do they stack and give him a bonus of 2?
Brass and Zinc Savants already get two bonus Nudges on rolls that use their powers (which is not an insignificant advantage), so I wouldn't give them additional bonuses to their stunts.
As to the Mistborn with both Tug Emotions and Push Emotions, I haven't seen anywhere that the bonuses wouldn't stack. It is like I was stating in response to the first question, but instead of rolling for a Challenge, the character is given an automatic Outcome of 1. Two Outcomes of 1 means two bonus dice.