If watching any sort of "LiveTV" programming, they will likely not be synced up. (each device off by a second or so.) If you try to watch both TVs like this, often will get an echo/stereo experience so normally just mute one TV and let video play.
If you want to simply "continue watching/resume" on-demand content on another TV, make sure the Roku devices are all on the same Roku account (email address). You can verify this in Settings/System/About.
Depending on the channel/app you are watching, you may have to look in the menu for a continue watching/resume section. Sometimes, just clicking on the same movie/show will simply provide a pop-up message box with the option to "Resume from hr:min" or "Play from the beginning" selections. Some channels/apps have no such feature so you are simply stuck with Fast Forwarding to your desired section.
For Roku Channel content, you can simply scroll down to the Continue Watching section and make your selection there.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941