MixBlend.Addwhere you should be using the default
MixBlend.Replace. I assume that you previewed the second track in Spine normally and did not switch the second track to
AdditiveMixing as described here, right?
If this is not the problem, then perhaps the looping first
shoot 1animation has not keyed the position/rotation of the yaw bone. This way the additive animation would add to a never reset value, leading to an ever growing offset.