I currently have UE 4.27.2 and UE 5.0.1 sources. If I register one or the other the Rider Unreal plugin will find and open ONLY projects from the last installed engine. Opening the .uproject file from the File->Open dialog results in a failure to load the project, with a message: Failed to locate Unreal Engine associated with the project file.
However there is no option or location I can find to specify which Engine source to use with a particular project.
This didn't seem to happen with the 2022 EAP version of Rider.