Jimmy, ‘dual key’ is used to describe a single apartment where two parts can be locked separately. The ones I’ve seen are effectively a one bedroom apartment plus a studio. So you can live in it as a two bedder, or close a hall door to create the one plus studio. Have a look at developments like Central Park and City Quarter in Sydney.
Believer: Unless otherwise noted on your strata plan or by-laws, entry doors and their locks are common property. Be wary of changing the lock, as fire regulations only permit certain locks. You can, of course, have the lock re-keyed, which is a straightforward job for a locksmith.