Sorry but you can't not with remotes you have now, which are IR remotes. Roku tv's from such brands Tcl, Insignia, Onn, Magnavox, Westinghouse, Jvc, Haier, Hitachi, Philips, and Sanyo all use the same IR commands.
You would need to abandon the IR remotes from both roku tv's and pair Wifi voice remotes to each. Even when the Roku tv is paired to wifi remote it still can accepty IR from other remotes.
Edmund Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.