The separate appliances option was something Trigeo did in the days before virtualization. When you were constrained by a physical chassis and the number of platters on a spindle, sometimes the only way to expand the appliance was with another box.
Now that virtualization, SANs and LUNs are a thing, the need for additional appliances has been largely replaced, and with the virtual appliance it's not a supported deployment. Besides, why have two VMs chewing up resources and adding latency to your logging when one beefy VM will do the job?