Combine 3D objects (Photoshop Extended)
Merging
3D layers allows you to combine multiple 3D models in one scene.
Once combined, each 3D model can be manipulated separately, or you
can use position and camera tools on all models simultaneously.
Open two document windows, each containing a 3D layer.
Make the source document (the file from which you are copying
the 3D layer) active.
Select the 3D layer in the Layers panel and drag it into
the window of the target document (the file which will contain the
combined 3D objects).
The 3D layer is added as a new 3D layer
in the target document. It becomes the active layer in the Layers
panel of the target document.
In the Tools panel, select a 3D camera tool.
In the options bar, from the Position menu, select the layer
name for the original 3D layer in the target file.
After
you match the camera position of the two 3D layers, the two 3D objects appear
together in the scene. Use the 3D object tools to reposition the
objects before merging.
From the Layers panel options menu
, choose
Merge Down.
The two 3D layers are combined into one 3D layer.
The point of origin of each model is aligned.
Note: Depending
on the size of each 3D model, one model may appear partially or
completely embedded in the other model after merging 3D layers.
After merging two 3D models, all meshes and materials from each
3D file are contained in the target file and appear in the 3D panel.
In the Meshes panel, you can select and reposition individual meshes
using the 3D position tools in the panel. See 3D Mesh settings (Photoshop Extended).

To alternate between moving all models at once
and moving individual models in the layer, switch between the 3D
position tools in the Tools panel and the tools in the Meshes panel.