Use geom_dl
to label additional layers:
WithLegend <- ggplot(volcano3d_1,aes(x, y, z=z, colour=..level..))+
stat_contour()+
stat_contour(data=volcano3d_2)
## direct.label labels the first colored layer.
SomeLabels <- direct.label(v)
## Additional labels can be added using geom_dl layers.
MoreLabels <- SomeLabels+
geom_dl(aes(label=..level.., colour=..level..),
data=volcano3d_2, method="top.pieces", stat="contour")