Final answer:
The IPS mode requiring more than two CP8 or CP9 processors on a FortiGate device is the proxy-based mode, which involves intensive content inspection.
Step-by-step explanation:
The specific IPS (Intrusion Prevention System) mode that can only be utilized with FortiGate (FG) devices equipped with more than two CP8 or CP9 processors is the proxy-based mode. FortiGate devices leverage these high-powered processors to handle the intensive workload that comes with the deep content inspection required in proxy-based operations. Unlike flow-based or firewall-based modes, which primarily focus on inspecting the headers and some aspects of the payload of network packets as they move through the device, proxy-based mode involves buffering traffic to inspect content more thoroughly, which can include full SSL inspection, and sometimes requires more processing power.