I also have this problem. In v8.5, Mesh>Mirror Geometry>Settings......there is 'Merge with the Original' selection and 'Merge Vertices' radio button.
When I have these selected and click Apply, it still merges more than 2 vertices (which I dont want). I've even tried modifying the Merge Vert threshod (via, Edi Mesh>Merge>Settings and changind threshld value to 0.00001. I've also ensured all my verts are snapped to the Y axis.

can anyone help, specifically with this 'Merge with Original' route?
Thanks
s6x