I've done a few of these dual-occ's for clients (in Blacktown LGA) and here's the problem:
Parking
SEPP: No additional parking needed
DCP: Need to provide (on average) 3 more parking spots, with turn-bays and visitor parking, 2 must be covered
Minimum Landscaping
SEPP: Generous landscaping controls
DCP: Not so generous and must be split between the 2x dwellings, often ruining separation/privacy between the 2x dwellings.
Max Size
Whilst it would initially seem that the DCP is more generous,
SEPP: 60 sq m max + can add garage/carport/patio etc to make it 120 sq m or so
DCP: Almost impossible to achieve the same floor space unless the land is >750 sq m
Other DA Costs
SEPP: Nil
DCP: DA Consent Conditions almost always add around $10,000 or more for beaurocratic things like engaging an Engineer 2 to 3 times, Surveyor 2 times, landscape architects, RTA Turn-bay schematics, hydraulic engineers 2x etc. Expensive approvals just ain't fun.
The DCP requirements and 'DA Consent Conditions' can add considerable cost and hassle not to mention delays (it's a DA)
All but 1 client has stated to me that they wished they'd listened to me and done a Granny Flat. Its just that Council treats 'dual occupancy' as a significant development and they hammer you with additional costs - they can be quite considerable.
The SEPP trumps dual-occupancy UNLESS you can subdivide imho.
Brazen.