1. Here's how
1.1. Let's say that Lumion Pro is already installed and that Lumion (Standard) has not yet been installed.
1.2. Rename the existing Lumion Pro installation folder to something else.
1.3. Right-click on the shortcut for Lumion Pro, select Properties and update the path to the 'Lumion.exe' file in the renamed Lumion Pro folder, so that it still starts Lumion Pro.
1.4. Run the installer for Lumion (Standard).
1.5. Click on 'Yes' when you see the message on the screenshot below (it will not be able to uninstall that version as you changed the installation folder name).
1.6. When it is done, update the name of the newly created shortcut to the 'Lumion.exe' file, so that you know that it refers to Lumion (Standard).
You can now run Lumion (Standard) and Lumion Pro via the two different shortcuts.