Cornish Jack is NOT correct.
In the 744 the "Master" FMC is the one selected by the switch in the upper center instrument panel -- usually the Left. While the FMCs update each other, only the Master provides actual guidance commands to the other systems. If the Left FMC fails, the switch has to be manually moved to Right to regain Autoflight (VNAV, LNAV, Autothrottles).
Independent of that switch, the PFDs and NDs are fed by the FMC selected on the respective Capt/FO Source Select switches -- usually Left for Capt and Right for FO.