If you map your factor new
to both colour and linetype, and specify the same labels for both scales, then ggplot will happily combine the colour and linetype legends automatically:
dec.dens1 = ggplot(dec_ema_df, aes(x=dec_ema, group=new)) +
geom_density(aes(colour=factor(new), linetype=factor(new)),
size=1, alpha=.60, kernel='epanechnikov') +
xlab(NULL) +
ylab(NULL) +
scale_linetype_manual(values=c('solid', 'solid','solid','solid',
'dashed','solid','solid','solid'),
labels=c('Dec `04','Dec `05','Dec `06','Dec `07',
'Dec `08','Dec `09','Dec `10','Dec `11')) +
scale_colour_manual(values = cbbPalette,
labels=c('Dec `04','Dec `05','Dec `06','Dec `07',
'Dec `08','Dec `09','Dec `10','Dec `11'))