My answer varies based on how I interpret the question, so there are three answers to be had:
If we work on having two copies of what one person on each planet can possibly have, then like most it's all about the TLR Fullborn (failing that, it's a tough choice but I'd go for a full Mistborn over a Feru) plus AonDor to add in teleportation, healing of others, and the ability to blast enemies away in the unlikely event that there's no metal to be found. If Aons can create food, presumably they can make metals, as well?
A step further is if we only take on what people can realistically achieve rather than the exceptional. In this case it's a tough one but it has to be double Zinc and Awakening. Think about it for a moment; stored mental speed to allow for extremely perceptive rioting, edging people towards handing over their Breath. Live becomes pretty cushy pretty quickly.
The last one, and the one I feel most accurately answers the question is to consider Twinborns an abomination of two magic systems (haha). With that in mind, it's all about the Soulbearer-Elsecaller. Store up as much Stormlight as your Soulcast crapton of Nicrosil can hold, and you're set way up until the next Highstorm. You can Soulcast anything your whims call for, you can travel the world (and others) until your heart is content, and you essentially heal constantly and would be functionally immortal.
Truth be told, I think the lack of super-overpoweredness makes the Soulbearer-Elsecaller that much more interesting. Dibs, and voted.