Your current TV remotes use IR control, so they control anything that is within the beam of the infrared signal. Almost all Roku TVs support using a WiFi Direct remote, which are discrete to the device they are paired to. For $20 each you could get two WiFi remotes and pair one to each TV. That's about the only way to control two TVs in close proximity to each other.
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.. or you could get by with just one new wifi direct remote, i.e. either of the Roku Voice remotes available from Roku or other retailers. Link it to one of your tvs and use the Voice remote to control that tv. Cover over the infrared receiver of that set with opaque tape to prevent it from being controlled by infrared signals of the original remote.
Continue to use the original infrared remote to control the other set.
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I have a similar setup. I have a room with one 65" tcl Roku TV surrounded by four 32" tcl Roku TVs. All five remotes work on all five sets. So far the only way I found to individually control a single set is to use the Roku app which let's me select the TV.
Since I really only want to be able to turn them on our off when I set them up, and since I already have Alexa devices at home, I named them TV-2, TV-2... in my Roku account. However Roku doesn't have groups other than rooms so in Alexa, although they all appear in my Living Room, I also created a bunch of virtual rooms so I could group certain screens together like Left Screens, Right Screens, The Wall, etc. This works fine but I know my control options are limited beyond this. There are other ways using Alexa and third party apps or skills that allow some other commands but the more complex you want to get you have to expect the setup won't allow everything.
I do also use Alexa to control a Fire TV stick which I have split and run to each set. This allows me to use less bandwidth streaming something when I want to watch one thing on more than one screen and simultaneously stream from another service or TV on the other(s).
Works fine for the most part but did take a while to set up and I had to make sure all of the sets had the exact same settings in terms of resolution, CEC ability, etc.
I set this up during COVID-19 while working remotely and can also use any of the screens as a monitor wirelessly with Windows 10 from a computer or laptop. I also have a separate mini PC behind the TVs so that I can use any of them for some basic things I need a computer for but want in the big screen such as video conference calls using any service I use on any other computer. Connect a webcam to the mini PC and I'm good. Hardly ever run into problems this way.
I also have them synced so that they all have the same channels installed.
Since they are in the same room I never pause on one set and resume on another since usually if I stop watching something and start later on another set of usually asks if I want to resume where I left off anyway. You should y lose this ability if you start creating separate accounts for most services. So rather than separate accounts I chose to create fake groups which Alexa treats like rooms (or vice versa). It did take a while to figure out because nobody from these companies really dumbs it all down for us. Plus, Alexa allows for a device to be part of more than one group and also nested groups. Not every service can handle this. So, for example, if Roku cannot, then use Alexa, but remember, Alexa may not be able to do certain Roku things.