I was able to pair two GearVR controllers using Windows 10 build 2004 and got them working perfectly in Driver4VR without having to reinstall windows or somehow downgrade to 1809 (is that even possible?).
I too was frustrated that I could not use a second GearVR controller due to them both using the same Identity Resolving Key (thanks Samsung), but also knew that if you had previously paired them prior to updating to Windows 10 1903 that you were able to continue using them so there must be a way to get the latest build of Windows 10 to allow both to be paired.
The solution is to temporarily change the IKN for one controller so that Windows 10 fails to see the conflict and allow the second controller to be paired, you can then change the first controller’s IKN back and still operate both controllers independently.
I am happy to provide a detailed set of instructions, but I would like to check with Greg first if posting instructions involving modifying the registry is permitted on the forum. If those instructions are not welcome, the above paragraph should give anyone with a reasonable knowledge of the registry enough information to figure it out for themselves.
See the attached screenshot showing the two controllers in Driver4VR.