Reflect & Glyph, how to add vectors at the symmetry plane


I have a small mesh with two cells. And some vector data at the points (nodes). I use Reader->Glyph to get the left figure.
Now for this mesh and the vector data Y Min is a symmetry plane. Thus, I use Reflect, more specific Reader->Reflect->Glyph and get the right figure.

Now, this is not what I want as also the vectors got reflected at the symmetry plane. At the three horizontal points/nodes in the center I would like to have only one vector at each node. That one vector at each node should be the sum of the two depicted vectors at each horizontal middle node. I tried to apply Merge Blocks but it didn’t help.

How would I do?