With regard to 30 minutes battery duration with the single battery NG...
In the case of a complete AC power failure in an NG (unlikely), I have always considered the 30 minutes of battery duration to be employed only during the approach part of the recovery....you know, when you really need all those instrument thingies and talky things to get you to the right end of the runway going in the right direction.
Why would you drain the battery(s) in level cruise flight, when there are quite enough indications of heading, altitude and speed, then have not enough left when you really need it, to shoot a normal(ish) approach (VHF1, NAV1, ADF1) if the need arose.
As an aside...well almost...in a dual battery NG,
1/ the secondary battery is not normally used for anything at all, the ONLY time the two batteries are connected is when standby power is activated, and
2/ if you ever arrive at the airplane to start the APU, and find the primary battery isn't up to it, selecting standby power on to connect the two batteries won't help, since selecting the APU to start is the only time, with standby power selected, that the two batteries are disconnected from each other.
Cheers,
ew73