Примерно так:
Mesh oneMesh = new Mesh();
Mesh otherMesh = new Mesh();
Mesh combinedMesh = new Mesh();
CombineInstance[] instances = new CombineInstance[2];
instances[0].mesh = oneMesh;
instances[0].transform = Matrix4x4.identity;
instances[1].mesh = otherMesh;
instances[1].transform = Matrix4x4.identity;
combinedMesh.CombineMeshes(instances);