To use it, while the script is running, you can right-button drag window borders, and the script will move all neighbouring window edges at the same time. The script will grab window borders within 10 pixels (configurable) of the border/s you are right-button dragging. It also assumes a seven pixel wide window resize border, but that's also configurable at the top of the script. You can resize from edges or corners - giving you lots of power when resizing windows.
The most common use-case I have is with just two windows positioned with Win+Left and Win+Right (like explained here). With this script you can resize both windows effortlessly.
Here's an example of it in action:
the window snapper script. Please let me know what you think after using it!
The script is licensed under a Creative Commons Attribution 4.0 International License.