I do different things depending on the phase of the flight...
* Below the transition altitude and outside the circuit, both are on QNH.
* Above the transition altitude, the "primary" altimeter (usually the one to the right of the AI) is on 1013, the "secondary" on QNH.
* In the circuit, the primary is on QFE, the secondry on QNH.
As for "standard" practice - no-one ever told me anything!
Cheers,
MD