So far having gone through the checklists, you would have attempted to resolve the situation by shutting down the engine, firing both bottles and the resultant would be operating the aircraft with a with single pack, having started the APU for electrics only.
Configuring for additional thrust would enable you more performance, this now requires you to pressurize the aircraft in a different manner, using APU Bleed Air, or remain unpressurized for the remainder of the flight.
Note: With the engine unable to get extinguished having used both suppressant bottles, the location of fire may be in such location as to be unable to get extinguished, but additional O2 into the area of fire may be very harmful.
My guess thus:
Opening the APU Bleed Air Switch may feed the fire with additional O2 forced into the ducts near the blaze location.
In addition I would tend to agree with framer that opening the APU Bleed Air Switch may feed contaminated and very hot air into the Cabin area which is not designed to withstand these very hot temperatures and may cause serious additional problems.