A Dual Inventory Fare is not the same as a fare that allows booking different segments of the itinerary into different booking classes. That is quite common and almost all long-haul fares allow it for short-haul connections. On BA, often visible as an I class business long haul fare booking into J class business for a connection on a short-haul flight. For example EDI-LHR-JFK-LHR-EDI. Also things like a business class short haul flight connecting to a first class long haul flight requires the use of a different, business class, selling code for the short haul flight.

When selling a ticket with a DIF fare, Dual Inventory is only required at sale time. So at the time you buy the ticket, there must be both availability in the cabin you will travel in the appropriate booking class (for example booking class T to get you into the WT+ cabin) and availability in some WT (economy) booking class, for example N. No N availability is taken away and unless you inspect the fare you will not know that N inventory was ever involved. The booking class remains T and you are booked into booking class T for all travel purposes.

If the availability on the flight was T3 and N1 you could book three such tickets.. But if the availability was T3 and N0 you could not book any of them. You would have to find some other, likely higher, fare.