According to Airbus, for A320.
The power supply split function that monitors the electrical segregation of the FMGC's does not consider the APU generator as an independent electrical source. As a consequence, when an engine generator is lost the capability is downgraded to CAT 3 Single.
The electrical power supply split function monitors......
-The two BUS TIE contactors
-The DC TIE contactor (between DC BUS 2 and DC BAT BUS)
When one of the above contactors is closed, the FMGC considers that the power supply split is lost (independently of the APU generator)
The A330 does not have this restriction and can be CAT 3 Dual capable even with one engine generator failure as APU is considered an independent electrical source.
Don't know why this isn't in the A320 fcom's.
Scally