They cannot be installed side by side, however one can be installed in PowerShell (non core), and the other in PowerShell Core.
From the comments in the docs you specified:
Question: The document says not to install the modules side-by-side but is there any reason that we cannot run AzureRM in PowerShell 5.1 and AZ in PowerShell Core on the same machine?
Answer: No, no reason at all. In fact we are thinking of suggesting this as an advanced scenario for those with a substantial investment in AzureRM scripts