Thanks for raising this.
The documentation is misleading in its wording (We’ll get this addressed). You cannot combine animation graphs directly because of the nature of the anim graphs themselves.
To note in the documentation of the skeletal mesh node of Mutable (https://github.com/anticto/Mutable-Documentation/wiki/Node-Skeletal-Mesh):
Anim Instance : This is metadata that will be attached to this skeletal mesh node. If a generated CustomizableObjectInstance/skeletal mesh ends up using this skeletal mesh, it will also have this anim instance metadata. It will be possible to query the present anim instances in the CO Instance via the Mutable API (grouped by component and AnimBlueprintSlot, the next property). The anim instance will not be automatically applied to the generated CustomizableObjectInstance/skeletal mesh, it will have to be done manually by the programmer possibly in the Instance Generated Callback.
The idea behind this current setup is to apply that metadata and use linked layers or slots to inject specific animation information for that section. Often you use a similar core set of joints that lead to your newly applied leaf nodes skeleton information, and then use a linked layer to apply that animation directly using blend by bone node in your animgraph. There are other creative solutions, but you cannot merge animation graphs like that easily right now.