Hello @Harald, thanks for your reply.
I set the preview animation as follows:
(Sorry, it is in spanish)
The layer 0 is Idle, layer 1 is Shake.
The desired result is more visible with another animation called "Fall" instead of Idle.
Here is the fall animation played alone on track 0:
And this is when "shake" is on track 1 as well:
However, setting these animations the same way in unity does not give the same result, it only plays the track 0 animation, (idle or fall, it doesn't matter). The expected result is what we are getting in the spine animation preview.
Both tracks are keying the rotation of the same bones.
I will try the MixBlend.Add mode in the meanwhile and come back if I get any result.
Thank you again, I hope this explanation helps you helping me!