Using classic Outlook for Windows in business environments
Unfortunately Lucas, this is a poor answer to the question. Just saying that "old Outlook" and "new Outlook" can be installed in parallel doesn't work. When new Outlook gets installed, it changes the association of .eml and several other Outlook file types to itself. This breaks old Outlook and causes users to put in HelpDesk tickets because they can no longer work.
There should be a way to prevent new Outlook from being installed on the machine via a group policy or registry key.