Setting two vertices to the same axis coordinate without movine one of them

I often have the case, where I have two vertices, and I want them for instance on the same z-coordinate. That can somewhat be done by marking the two vertices, then use scale in x axis with 0 … i.e. keyboard sequence s, z, 0, <enter>.
This will even the vertices out on their Z coordinate, but it will move both vertices along the Z axis, one a bit up, the other a bit down … and I want to avoid that … I want only the second vertex I clicked to adjust its Z coordinate, using the first as reference … and not touch the location of the first Vertex at all, it should remain, where it is. How does one do that?

Set your pivot point to active element (on the 3d view header, hotkey is alt+period), select the vertices so that the one that is the reference as last (active selection) and then scale to 0 along axis.

Select the vertex at the correct z coordinate, Shift+S / curser to selected. Set pivot to 3d curser, select other vertices and scale by zero on the z axis
or
Set the pivot to active element. Select the vertices you want to move, shift select the vertex at the correct location and scale by zero on the z axis
or
Enable snapping set to vertex, select the vertex you want to move, move on the z axis and snap to the vertex at the correct location