I didn't think Hawk was talking about that line that normally occurs in this case
ops:
Anyway he also can combine the surfaces (2 on 4 sides) and then delete the four vertices in the middle of the new box ... because your 2 solutions do not work for me ( :?: )
I mean, if I delete the right side of the box before mirroring, I still have the middle line after the mirroring process, and your second solution only unselect the front (and back) surfaces ... top, side, and bottom surfaces are still selected, so if you delete at this point, you end up with a very strange box (??).
8)