My SPF, both fans come on at the same time. Turn the fan switch located on the dashboard and see if they both come on. If not, you have a bad ground (maybe not, because most grounds are grouped together on SPF's) or a bad motor, a bad fuse or a bad relay. Another thing could be just a bad connection at the fan. Pull off the pigtail and see what it looks like. Clean both ends and reinstall the connector.
I'm assumining you have your owners guide that shows the fuses and wiring diagrams.
Edit - my SPF has dual fans, each with its own fuse and relay, with the optional fan switch on the dashboard.
Also, you can get a great wiring diagram from Blas on this site.
Wiring diagrams for ALL Superformance series MKIII cars are now available....