Hi, TM.
I am a little confused – do you plan to build one duplex (consists of 2 units) or two duplexes (consists of 4 units)? cos I  think the "2 Dwellings on one allotment" usually means dual occupancy – whether attached or detached. One duplex consists of 2 units will be regarded as 2 dwellings attached to each other in council's point of view. So two duplexs (consists of 4 units) will be considered as 4 dwellings by council and thus are difficult to get approval in a low residential zoning.