I haven't heard the term "duel link" before with DVI. But I would guess that since there is a DVI-I standard in which both analog and digital are supported over a single DVI cable, that this might be what they are talking about.
If so, I think that the 811 is DVI-D (digital only) so I think that the "duel link" DVI-I cable might be overkill (but should still work anyway).
Just a best guess here.