Need to process half mesh of a gameobject, after that then merge and output back to unity.

Submesh or half of a mesh? Two different things and two different potential solutions.

If submesh, then these will have primitive attribute ‘unity_input_mesh_name’ with name of the mesh. It will have different material path as well ('unity_material')

For half of a mesh, not much you can do on the Unity side, unless you write your own input mesh uploader. On the Houdini side though, you can use a boolean or other SOPs to get at the subset of the mesh geometry.