The non-voice remote, if it's one that came with the set, likely controls the tv via direct line of sight infrared (IR), which will control ANY Roku TV. If the two sets are both where the IR can reach it, it will control both of them.
[Edited later to add:
Exception: Roku TVs from Sanyo and HiSense respond to both the standard IR command set mentioned above and to their own separate command Sanyo/HiSense command set. The remotes from Sanyo/HiSense only emit the Sanyo/HiSense contol set and will not control Roku TVs from other manufacturers..]
The voice remote controls via point-anywhere wifi-direct that is linked to just the TV it is controlling
So, find the IR receptor on the front of the voice-remote-controlled TV and block it, perhaps with some opaque tape. You may have to experiment a bit if the location of the receptor is not visually obvious.
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