Out of curiosity why are you using Mirror Cut if you are just trying to mirror and attach the geometry?? You have a lot of different options open to you e.g. Mirror geometry, you could duplicate the object yourself then scale it negatively along one of the axis (Note if you do this you will have to reverse the direction of the face normals. Another useful tool is Edit > Duplicate Special > Options. Its basically the same method as above but you set the options in the Option window and it reverses the normals for you. The only problem with doing the latter two is that you will have to combine the objects together using Mesh > Combine then merge all the central vertices. I would recommend trying the Mirror Geometry tool first. Good luck with the assignment and I hoped that this has helped.