With one or both using IR remotes, the only way is to abandon both IR remotes and switch each Roku TV to Wifi voice remotes.
There's no way to stop it with your current remotes as they appear to be IR (infrared). You could get one of the Voice Remotes and pair it to one of the TV's and then cover the sensor on that TV so the Voice Remote controls one TV and the IR remote controls the other. You could accomplish the same thing by using the Roku app on your phone or tablet as a remote.