You can log into the FortiGate-6000 management board CLI or the FortiGate-7000 primary FIM CLI and use the following command to force one of the FPCs or FPMs to always be the primary or master FPC or FPM:
execute load-balance slot set-master-worker <slot>
<slot> is the FPC or FPM slot number.
The change takes place right away and all new primary FPC or FPM sessions are sent to the new primary FPC or FPM. Sessions that had been processed by the former primary FPC or FPM do not switch over, but continue to be processed by the former primary FPC or FPM.
This command is most often used for troubleshooting or testing. Since it is not a configuration change, if the FortiGate-6000 or 7000 reboots, the usual primary FPC or FPM selection process occurs.