Etymology: alluding to engagement in sexual activity of auxiliary orificia (e.g., mouth, anus) in addition to or other than the orifice devoted to elimination of sloughed endometrium, aborted embryo, babies, other emissions, excretions or discharges (i.e., vagina or cunt). The generic use of "double-cunted" represents semantic bleaching or desemanticization (cf "bastard" as a general insult though it originally refers to illegitimate children)
"Please use breath mints or shut the fuck up double-cunted cum-guzzling gutter-slut!"
"You back-stabbing, double-cunted bitch!"
It can be especially attributed to somebody who has acted in such a way that their cuntworthiness is doubled, therefore justifying such a title.
"Ouch, I have just been double cunted"