We purchased a Roku Premiere+ earlier this year to assign to one TV downstairs, and have a TCL tv that also has its Roku remote.
Everything with this configuration worked fine. However, the TCL's internal network card went out and while a bit M.C. Escher I went ahead and purchased a Roku Express 2019 to connect a Roku to my Roku. 😆
Anyways, I just set up the Express HD and it turns out that its remote also controls the Premiere+.
Interestingly enough, the Premiere+ remote does not control the Express HD.
I do not see anything obvious to change up the frequencies on the remote.
Re: Roku Express HD Remote is Controlling Two Rokus
Sorry, even though the premiere + came with wifi remote, it still responds to the Roku IR code set that Express is only using. You can try taping up front of the premiere + so it doesn't accept any IR commands., it will still respond to Wifi roku remote it came with since signal goes through walls and tape.
Or if your roku express is either 3900 or 3930, you can purchase a wifi remote and pair it to the express, so neither will be using an IR remote and so there won't be conflict.