Animation of what i mean with that title
I tried the origins only but that didnt work at all.
Please only 2.8 solutions so that it becomes easier to everyone to understand it.
If I understand correctly, you want to move the selection of vertices in opposite directions on the X axis without deforming the scale, right? If so, do it like this ...
In the Sidebar, select the Push / Pull tool.
In the Header, in Drag, choose Tweak to activate the tool's gizmo.
In the Header, in Transform Pivot Point, choose Individual Origins.
Then just drag the gizmo. The vertices will move without deforming the scale.