Below transition altitude for a x-country flight I'd take off with QNH on the primary and QFE on the secondary. For cruise below transition I put QNH on both, above transition 1013 on the primary and local QNH or RPS as appropriate on the other. On approach I use QFE on the primary and QNH on the secondary unless the published circuit level is given as an altitude in which case I reverse (means I always fly the level on the primary regardless of what type of level it is).
Unless of course I'm flying CB and don't want neck ache
MC.