I'm not sure what you mean by "relearn", but if you have two voice remotes you should be able to pair either one with either Roku device. If you can't get the light to flash by either holding the pairing button for 5 seconds or simultaneously holding the Home and Back buttons (the two buttons next to each other at the top of the remote) for 5 seconds, the remote may be dead. You can also try holding Home and Back for 10 seconds which supposedly resets the remote.
A remote can only be paired with one device at a time, so if you want to move a remote from one device to another and back, you'll have to pair it again when you move it back. My preferred way of pairing a remote is to put the remote into pairing mode and then connect the power to the Roku device. When pairing a remote to a different device it's helpful to disconnect the power from any other Roku devices so they don't interfere with the pairing process.
How to set up your Roku® voice remote | Official Roku Support
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.