Not sure if Z-Brush does what you want with an "overlaid" texture UV like Mike's. By "overlaid", I mean how head uses the whole UV space, as does body, so when you look at it in UVMapper, you see them superimposed one over the other. Z-Brush may end up painting on to both head and body, where they are superimposed. Still might be useful, if you paint details (scars) just near the seam, then use a paint program to separate into the two textures, then copy those details on to your main texture.