Chris, Lyman,
The "235 V AC distribution" consists of 4 buses, L1, L2, R1, R2. Normally each is powered by one main engine generator, but there seems to be some crossbar switching, so if generators fail, the associated buses are powered from other buses.
The APU can power all four buses, or those buses not powered by main engine generators. As far as I can see, in practice, the buses can be switched onto different generators dynamically, as load and available generation requires.
They are all variable frequency AC, depending on generator rpm, so I guess it's not a good idea to try to power one bus from two different sources at the same time.
(I don't know how authoritative my source is. It says "do not use for flight" and appears to be from 2010.)
Bernd
Last edited by bsieker; 23rd Jan 2013 at 14:37.