In flight you won't be
able to put the APU on both busses, it's not possible. With a dual generator/engine failure don't forget that most essential equipment will be powered through the transfer and standby busses, which will both remain powered with the APU powering just one main bus - you'll just lose the high load and non-essential items on the other bus i.e one elec hydraulic pump, a couple of fuel pumps, some pitot heaters and a few other bits.
A good dual engine failure briefing can be found on Chris Brady's B737 site, written by Capt Mike Sykes of bmi:
http://www.b737.org.uk/lossofthrust.htm
This may go some way to answering your questions about use of the APU in tis scenario.