C+
I dont work on helis (so maybe I should just shut up now) but on corporate aircraft the No.1 nav is hooked to the emergency busbar (battery), as is the no.1 comm/stby inst setc.
I believe that the FD V-bars would not be a requirement to work under emergency conditions (double genny failure) anyway and just the attitude (No.1 AHRS/VG), airspeed (No.1 ADC), altitude (No.1 ADC), heading information (No.1 compass) and other essential services required for the 1 hour duration to get you down safely.
Possibly Nick L. can enlighten the reasoning.
Regards
NigD