A Microsoft support document explains how you’ll be able to run the old Edge and the new Edge together if you want. The bad news? It requires it requires Group Policy, which leaves out Windows 10 Home users. (There are Registry-based workarounds, of course, so I’m sure we’ll find out what that is before the new Edge launches.)
“Installing the Stable channel of the next version of Microsoft Edge at system-level will cause the current version (Microsoft Edge Legacy) to be hidden,” the support document notes. “If you want to let your users to see both versions of Microsoft Edge side-by-side in Windows, you can enable this experience by setting the Allow Microsoft Edge Side by Side browser experience group policy to ‘Enabled’.”
To enable this, open the Group Policy Editor, navigate to Computer Configuration > Administrative Templates > Microsoft Edge Update > Applications and select “Allow Microsoft Edge Side by Side browser experience.” Then, click the Edit policy setting link, and in the window that opens, select Enabled and then click OK.
Microsoft also notes that the Allow Microsoft Edge Side by Side browser experience should be enabled before the new version of Microsoft Edge is deployed for the best experience. If you don’t do it ahead of time, you’ll have a few issues to deal with. For example, you’ll need to run the new Edge installer again before you can both versions side-by-side. You’ll have to re-pin Microsoft Edge Legacy to Start or the taskbar. And any sites that were pinned to Start or the taskbar for Microsoft Edge Legacy will be migrated to the new Edge.
Given this, I expect a simpler solution for consumers to appear as well, so stay tuned.