Well the way album art works currently, there would be no way to display two disctint pictures simaltaneously. The way I see it you have these options.
1. Use a naming convention that shows songs w/ adult content. Example, filename in ALL CAPITALS.
2. Use parental Advisory label instead of album art.
3. Make sure album art had parental advisory label on it.
4. Use parenthesis in the file name, Example, "2Live Crew - Face down *** up (Adult Lyrics).