A Dual Key Concept Property is one which (usually) has a self contained studio accessed by a door, inside the main apartment. There is a common hallway, but separate lockable doors to each section. So the title belongs to one owner, but they can rent out the each portion separately, or live in one and rent the other.
The advantage is higher yield and/or income. You could live in the main part and rent the smaller part out, so you get income to help with the mortgage. Or let both parts out separately.
Your elderly parents stay in the studio, while your wife and kids stay in the 2BR or 3BR next door.
For Young Investor:
Rent out the studio to working professionals, yourself stay in the 2BR unit; or vice versa, when you are still single, you stay at studio, rent out the bigger space to small family.
At first, you rent out the smaller unit, but when your kid grows up, they request for own privacy, and you as their parent you don’t want them to stay far away from you, let them stay next door, yet their privacy are assured.
See the below example for Dual Key Concept Apartment