Are you using the SkeletonMecanim component with an animation Controller? Then having the belt animation on a separate animation layer should be the way to go.
If you are using SkeletonAnimation, you could use tracks at index 1 for the belt animation:
int trackIndexBelt = 1;
skeletonAnimation.AnimationState.SetAnimation(trackIndexBelt, beltAnimation, loop);
Alternatively, you could deal with sorting by adding a SkeletonRenderSeparator
component, as shown in the Spine Examples/Other Examples/SkeletonRenderSeparator
example scene. It demonstrates how you can split one skeleton into multiple render parts for depth sorting and rendering of other elements between them. However I would rather recommend using animation tracks or layers as described above.