Like Batman & Robin in the comics, a Super Duo is a pair of characters that may be perceived to be more than the some of their parts. These pairs of Supremes often have rules that directly reference the other member of the pair, but Super Duos are different - play them together in close proximity and they simply get better at what they do.
First up for consideration is fittingly the originally referenced Super Duo: Solar and Stalker. For a long time after Solar's release in the original Hero Starter Set we did not know anything about Stalker except that he formed a Super Duo with his feline companion, and what the benefit would be for doing so.
Super Duo works on close proximity as I have stated - it is determined by Spirit Trait in inches. For Solar and Stalker they each have spirit Trait 3, so within 3 inches of each other each adds +1 to their Agility as this is listed on their cards.
The upshot is that they become that much harder to hit with many ranged attacks when close together. For Stalker it is a free defensive boost that can also be paralleled by using his Elusive* Action to render him much more durable when needed - say when holding an objective for example. Boost that up further with buffs such as Solar's Touch of the Sun and then you have a pair of models ideal for holding out on an objective or offering area denial with added capability from their offensive Actions when needed. With the 3 inch range of the buff, they could be placed either side of an objective Token to limit access by enemy models while retaining their defensive boost.