Notifications are useless if I don't always see them. I can't know which screen I'm going to be looking at, so they should appear on both screens. Is there any way to get this to happen in Ubuntu 18.04?
In lieu of being able to find a better solution after dealing with this problem myself, I've made a workaround, though it's not precisely what OP asked for (it only shows a notification on the screen your cursor is on). It shouldn't be too difficult to hack other functionality into this, and it's short enough that everything it does can be quickly perused to verify it doesn't have any malicious code [one python file, and a one-line bash script], in contrast to the forks of notify-send that other people have made).
It listens for calls to notify-send (without intercepting them), and prints out the call's message on an image. The image will (should) popup on whatever screen is active, and close itself after 5 seconds.
This also keeps a log of notification messages, in a text file in the folder from which the command is run.
git clone email@example.com:hillcjames/NotificationAmplifier.git cd ./NotificationAmplifier sudo apt-get install python-pip pip install opencv-python pip install Pillow sudo apt-get install wmctrl sudo apt-get install xdotool
python ./amplifyNotifications.py, and just let it run.