The Yeah Yeah Yeah's songs also have the 2nd guitar charted to bass.It DOES happend...Just not often.
As long as there ISN'T bass in the song, charting something else to bass would probably only get voted down by people who think Rock Band is supposed to be realistic.
Jeez, why do all these babies want their rhythm guitars? All they do is maintain the riff while the lead guy does his thing. Can anyone name a song where the rhythm guitar gets awesome while the lead guitarist is performing a solo? Of course you can't! He's to busy keeping a riff you probably heard in the bridge or chorus audible! You're probably to distracted by the possibility of playing those precious chords to notice! That's all it's really about, chords.
I want to make the guitar charts more boring by splitting the one track between rhythm and lead
I don't think it would be a problem since back when HMX was making Guitar Hero games often times the "bass" would be a rhythm guitar because that was what was available or in some cases simply more fun than the bass on that particular track.
I always saw the "bass" option in these games to mean "rhythm string instrument" whether that's Bass (99% of the time) or Rhythm guitar (occasionally).
I know, I know what to do, OP. Tell them you can chart it that way, but only if you alter the rhythm guitar audio track to sound like bass