These two KB articles came up after a quick search. I' ve used the first one myself. They can be a little opaque at times, but the first is generic, and uses two FortiGates in the examples. You could just as well use one FortiGate with two VDOMs, if this is really something you want to shoot for...
Technical Note: Sample configuration of FortiGate WCCP server and client (traffic redirector and transparent proxy), with verification and troubleshooting procedures
http://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=FD32926&sliceId=1&docTypeID=DT_KCARTICLE_1_1&dialogID=63662107&stateId=0%200%2063664072
WAN Optimization, Web Cache, Explicit Proxy, and WCCP (PDF Document)
http://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=fortigate-wanopt-cache-proxy-40-mr3pdf&sliceId=&docTypeID=DT_PRODUCTDOCUMENTATION_1_1&dialogID=63662107&stateId=0%200%2063664072
Regards,
Chris McMullan
Fortinet Ottawa