Dear Blknotes,
I think that you are referring to the second song in the video called "G 70 new tones"; from that video it looks like the acoustic guitar multisample has fret noises on the left side of the keyboard, while what you hear when the demonstrator plays on the far right of the keyboard (at the end of the song) are the slide-notes which can be heard only hitting the keys very hard (in other words, there is a velocity split between "normal" guitar sounds and "slide-notes", just like Yamaha has done in the Tyros).
But this means also that, if you did choose that sound to play a song, it would be very difficult to assing another sound to the left side of the keyboard, because the guitar alone would take almost the entire 76 keys.... however I guess that you could still trigger the chords with your right hand, and at the same time be free to play the melody. To do this you would have to assing the chord recognition to the whole keyboard and at the same time choose a mode where at least three fingers are required to identify a chord: this woule leave you free to play single notes (the melody) between one chord and the following.