kentek , only TV's have inputs, so only they have an option on the Home Screen (or within the menu) to switch inputs. The TV functions on Roku remotes are limited to controlling power and volume. Sorry, but you'll need two remotes or one universal remote that can control both the TV and the Ultra.
There is one possible one-remote solution - set up the TV to always power on to the cable box's input (if possible) and use 1-touch play found under Settings->System->Control other devices to switch the TV to the Roku's input when you press a button on the Roku remote. So after using the Ultra, quickly power the TV off and on to get it to switch to the other input. For all of this to work, the TV has have a setting for its power-on input and it has to support CEC to get it to switch to the Roku's input.