Nate yeah, that's what I tried in the first place, however, I wasn't happy with the result since I was getting a clunky transition between the previous animation (let's call it "shaking") and the mixed "idle" + "stunned" animations. Looking for possible ways to fix the problem, I found the MixBlend but it didn't work as I thought.
Researching again, I've read this thread https://es.esotericsoftware.com/forum/d/16533-no-mix-at-a-higher-track-sometimes/6 and it seems to be the final answer to my issue! So just in case somebody needs it, here's my code to blend 2 animations on different tracks smoothly:
_skeletonAnimation.Skeleton.SetToSetupPose();
_skeletonAnimation.AnimationState.SetAnimation(0, "idle", true);
_skeletonAnimation.AnimationState.SetEmptyAnimation(1, 0.2f);
TrackEntry track2 = _skeletonAnimation.AnimationState.AddAnimation(1, "stunned", true, 0f);
track2.Alpha = 0.5f;
Thanks all of you for your help! @Nate @Harald