From the manual talking about the STC:
Use of Secondary Instruments: The original type design approved instruments for airspeed, altitude and vertical speed remain the primary indications for these parameters.
If the G5 Electronic Flight Instrument is installed in place of the rate of turn indicator, the original type design approved instrument for attitude remains in the primary indication for attitude.
If the G5 Electronic Flight Instrument is installed in place of the directional gyro, the original type design approved instruments for attitude remains the primary indication for attitude.
NOTE:For aircraft approved for VFR-only operations, the G5 Electronic Flight Instrument may be installed as an attitude indicator and rate of turn indicator.
I'd be surprised if an A&P would sign off on dual G5s without the opposing supporting instruments.
You might want to ask your A&P or avionics guy if they would sign off on a non-TSO Turn Coordinator install. It usually is less expensive and the instrument will probably work just as well as a TSO version. That instrument is rarely used except in an emergency.
Brian