I thought I could just open the group policy editor and enable the option "Allow Microsoft Edge Side by Side browser experience", but it is no longer there!
Unless you installed the required policy templates the required policy was never on your system.
- Install the Policy Definitions from Microsoft Edge for Business.
- Copy msedge.admx and msedgeupdate.admx to the
- Copy msedge.adml and msedgeupdate.adml (from the appropriate language/locale directory) to the
- Under Computer Configuration, go to Administrative Templates>Microsoft Edge Update>Applications.
- Under Applications, double-click Allow Microsoft Edge Side by Side browser experience.
- Select Enabled and then click OK.
However, this group policy must be enabled before Chromium Edge is installed, if it wasn't then additional steps must be performed.
For the best experience, the Allow Microsoft Edge Side by Side browser experience should be enabled before the new version of Microsoft Edge is deployed to your users' devices.
The installer can be run directly or automatically when the new version of Microsoft Edge updates.
- Allow Microsoft Edge Side by Side browser experience won't take effect until after the installer for the new version of Microsoft Edge
is run again.
- Microsoft Edge Legacy will need to be re-pinned to Start or the Taskbar because the pin is migrated when the new version of Microsoft
Edge is deployed.
- Sites that were pinned to Start or the Taskbar for Microsoft Edge Legacy will be migrated to the new version of Microsoft Edge.
Source: Side-by-side experience with Microsoft Edge Beta Channel and Microsoft Edge Legacy
I would take the appropriate backup of your system, download the Edge installer, and that should allow the required policy to take effect. This work around to your problem is required due to the fact, KB4559309 cannot be installed, at least that is according to Microsoft's documentation on the subject.