installing SQL SERVER 2016 side-by-side SQL Server 2012
Installing SQL Server 2016 side by side with an existing SQL Server 2012 can (usually) be installed without issues and is supported as they state in that article.


  SQL Server supports multiple instances of the Database Engine,
  Analysis Services, and Reporting Services on the same computer.


What you can't do however is using SQL Server Sysprep to install different versions side by side.

SQL Server Sysprep is basically an image based deployment solution like you have Sysprep for windows images.

So as long as you manually run through the installation wizard you are fine, but you wouldn't be able to install a SQL Server image using a prepared image by selecting this option on the advanced tab: