Try this:
1. Use Duplicate/Instance. This will make a copy that automatically updates with the orignal. FYI: Instance copies save memory.
2. Change the X scale to -1 if looking at front of head in front view. If right view use -1 for Z scale.
You should now have a new half that will update automatically when you change the first.
To the the halves together without a seem.
Using a surface model:
1. Move the halves (just a little) away from the parting line, so you can fit a new surface between them.
2. Use Dupicate/Expand to drop the istance to regular geometry (or just copy the instance).
3. In the Perspective View rotate the model so you have a good view of the adjacent edges.
4. Use Draw/Merge surface. Click on the edges to join. You should see a purple highlight on the edges. This will get both halves together by using a smooth blend between them. Disgard original surfaces.
If there is still an obvious bump where the seam was:
1. Use Edit/Add Point to add an extra row of points just to the left and right of the parting seem points. This may not be needed.
2. Use Edit/Delete Point to delete the rows causing the bump at the seam.
3. Use Edit/Add Point again to add another row in the middle if needed. This row should be smooth now.
I highly recommend practicing the merge surface command. You will find it very useful for a lot of shapes. Every surface has a U side and a V side no matter how complex the shape is. Merge Surface simply joins the sides you ask, but it takes some practice visualizing what you want.
Hope this helps. If you need more details or have more questions, just let me know.
Urp