You can assign multiple keyboard shortcuts (keybindings) for the same command using
gsettings command line.
One important thing to know is that Ubuntu 18.04 Settings GUI only shows the first keybinding for a command, so if you have multiple keybindings for a command, the others won't appear in Settings. You can use
gsettings to all the keybindings.
Let's say I want to add another keybinding for "Switch to Workspace 1". The default for me was
Super+Home, but I want to add a second keybinding
# list all keybindings
gsettings list-recursively | grep -e org.gnome.desktop.wm.keybindings -e org.gnome.settings-daemon.plugins.media-keys -e org.gnome.settings-daemon.plugins.power | sort
# confirm no other keybinding conflicts
gsettings list-recursively | grep '<Control>1'
# set multiple keybindings for "Switch to Workspace 1"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-1 "['<Super>Home', '<Control>1']"
# confirm value is set correctly
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-1
Now you can use either
Control+1 to Switch to Workspace 1. Remember, you will only see the first one
Super+Home in the Settings GUI, but it will work!