You certainly can "link" all your Roku devices to the same Roku account by using the same email address during setup and activaction. (The email address is your Roku account). In order to view the email address registered to your Roku account, go to Settings/System/About.
When the devices are linked, all your channels are synced between devices on that account. Adding or removing a channel will add or remove them from all linked devices. You will however have to enter the login credentials (at least once) on each device for channels/app requiring authentication.
To link your RokuTV, just use the same email address as your Ultra. If you have already setup your RokuTV with another email address, you will have to Factory Reset it (press and hold Reset button for 30+seconds or go to Settings/System/Advanced System Settings/Factory Reset). Then set it up and activate it again under the same email address as your existing Ultra.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941