Resonance is the combination of Surges amongst Surgebinders which has unique effects. In essence, it is a branch of effects which manifests within a person who bears at least two powers. The effect does not directly affect how the powers creating it are used. It's the interaction between, and side effects of, the combination of two or more magic abilities, which don't necessarily have to be from different Shards or even Systems. This has been confirmed by Brandon and it is Cosmere-wide mechanics when a Nahel bonded individual has access to two different powers.
Readers don't yet know the Resonances of other Orders of Knights Radiant.
Q&A with BrandonEdit
Q. Shallan has this awesome Memory thing going on, Jasnah has this geolocation thing, Kaladin is a really good fighter - are those just their traits, or is there something supernatural going on?
A. There is something supernatural about those. Each Order ... Well, how about this. If you look at scholars' interpretations, there are some scholars who think that these things are not supernatural, and some who think that they are. But, if you look, many Lightweavers had powerful mnemonic abilities.
Q. So it's definitely tied to the Orders?
A. It's tied to the Orders. Now, I am not going to say that you've got them all 100% correct, but each Order, there are things that come with Order, things that do not add up from simple the "you get this power plus this power," there is something else going on. And I would say that for Windrunners, watch the number of squires and the power of the squires... these are abnormal for the Windrunners.
Q. And each Order's squires are somehow different from the other Orders'?
A. Yeeeaaaa ... some Orders do not have them.
Q. But some have more?
Readers might thus extrapolate that a Skybreaker's gravitational bindings are going to be different from those of a Windrunner. Postulating about the combination of Gravitation and Adhesion, given a Radiant adhering gravity between different things, one thing will attract another thing (using gravity). Skybreakers may actually cause objects to repel with their Lashings, and they may fly by essentially removing their connection to gravity and then repelling from a surface. One force is the same, but their interaction with it is totally different because of how the two Surges combine.
"Merging" seems to be a key word for Resonance. A twinborn is going to have little to no Resonance if he or she is very new to their abilities. They're essentially going to be using them as individual powers. However, since Wax is at or approaching savant status with his abilities, they've started merging at a spiritual level. This merging brings out the Resonance between his abilities and allows him to bend trajectories with precision.