A FortiGate-7000 will continue to operate even if one of the FIMs or FPMs fails or is removed. If an FPM fails, sessions being processed by that FPM fail. All sessions are then load balanced to the remaining FPMs. Sessions that were being processed by the failed FPM are restarted and load balanced to the remaining FPMs.
If an FIM fails, the other FIM will continue to operate and will become the config-sync master. However, traffic received by the failed FIM will be lost.
You can use LACP or redundant interfaces to connect interfaces of both FIMs to the same network. In this way, if one of the FIMs fails, traffic will continue to be received by the other FIM.