It is impossible to install volume licensed versions of Office 2016 and 2019 together on the same PC.
The two main things that determine whether Office 2016 and Office 2019 can be installed together on the same computer are the version and the installation technology used to install them.
Office Professional Plus 2016 is Version 16.0 and use MSI installation technology, volume licensed Office 2019 is also Version 16.0 but use C2R installation technology.
According to the article "Supported scenarios for installing different versions of Office, Project, and Visio on the same computer", you can't install two products together that have the same version but use different installation technologies.
As a workaround, it's better to create virtual machines, install one of these Office 2016 and Office 2019 on them, then install another one on local PC.
Hope the information is helpful. Please feel free to post back if you need further assistance on this issue.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.