Combine meshes in Unity.

• Create a new Unity3D project. Call it CombineMeshTest
• Create an empty GameObject and rename it to app
• Select app
• Add component MeshFilter
• Add component MeshRenderer
• Set the mesh renderer material (Mesh Renderer, Materials, Element 0) to Default material
• Add New Script and call it App
• Doubleclick App to open in Visual Studio
• Replace the generated code with:

using System.Collections.Generic;
using UnityEngine;

public class App : MonoBehaviour
{
    public Mesh Mesh1;
    public Mesh Mesh2;

    private void Start()
    {
        var mesh = CombineMeshes(new List<Mesh> { Mesh1, Mesh2 });
        GetComponent<MeshFilter>().mesh = mesh;
    }

    private Mesh CombineMeshes(List<Mesh> meshes)
    {
        var combine = new CombineInstance[meshes.Count];
        for (int i = 0; i < meshes.Count; i++)
        {
            combine[i].mesh = meshes[i];
            combine[i].transform = transform.localToWorldMatrix;
        }

        var mesh = new Mesh();
        mesh.CombineMeshes(combine);
        return mesh;
    }
}

• Switch back to Unity
• Import mesh1 and mesh2 into the project.
• Drag the meshes into the App script:

• Press play to see the result.