Ok, here is what i would do aard, this is step by step to make it easy for you.

1. Select the half of your head
2. Figure out what axis is running through your head (X, Y, or Z)

3. Duplicate your head (Ctrl+D)
4. Open the Channel box (Display>UI elements>Channelbox / Layer editor)
5. Where it says "Scale _" (in the blank put your X, Y, or Z from step 2) make that -1.
6. Select both halves of the head and Combine (Polygons>Combine)
7. Click on your face and go into Edges (Rightclick - Edge)
8. Region select all of the middle edges and deselect all the extras (region selecting is the box when you drag with your mouse holding down ).
9. Once you have all of the middle edges selected merge the edges (Edit Polygons>Merge Multiple Edges)

Now you can go into your Vertex mode and move the verticies around as you please, seeing it is truly 1 piece now .