Combining two mesh doesn't work

I created two mesh runtime and I want to combine them. They are not attached to any game object.
with the following code:

          CombineInstance[] combine = new CombineInstance[2];
          combine [0].mesh = obj1_mesh;
          combine [0].mesh = obj2_mesh;

          Mesh combinedMesh = new Mesh ();
          combinedMesh.CombineMeshes (combine);

I get nothing. All the vertecies in the combinedMesh object are (0,0,0). Can anybody tell me what the problem is?

There is a second and third parameter for combineMeshes. Second is a bool for combining submeshes, third is a bool for using matrices. Also what kind of Object is obj1_mesh?

Also you are assigning borh meshes to index 0 in the array.