In two parts (4)
'in two parts' is the definition.
(in two parts)
This is the entire clue.
(Other definitions for dual that I've seen before include "Double" , "Consisting of pairs" , "Road with central division" , "Twofold" , "Consisting of two parts" .)
(Other definitions for duplex that I've seen before include "Having two parts, perhaps house" , "House with two units sharing a common wall" , "Apartment on two floors" , "Two-fold" , "A kind of flat on two floors" .)