I fear that it might be a bit more complicated than that. The BS1 and the Avant remember the last "TV" source that was used. Also, you could have even more than 2 sources on the TV trigger -- for example, an antenna source, cable source, and V.AUX.
To complicate matters even further, you must quickly press "TV" twice to cycle to the next source. Pressing "TV" just once has the system look for the video signal again. This is sometimes necessary in the case of an HD DVD player or BD player because the start-up takes so long and the system does not detect the video signal right away. Therefore, it might be necessary to have a single "TV" command on the Beo5 as a video source "refresh" command.
I suppose that having multiple video sources toggled on the "TV" button was seen as simpler on the Beolink 1000 or Beo4 rather than using the "list" button to pull up the "V.AUX" soft button; however, this poses a problem with the more versatile Beo5.