When you are using normal 30A or 40A relays it's a good ida to use one relay per fan.
Fan's draw a lot of current and the connectors and relay's can become very hot.
I always use high amps relays from Bosch: 0 332 002 156. With such a relay you only need one. It may seem to be overkill, but this relay will work forever.

It's a good idea to use the thermo switch in the radiator parallel with a switch and/or control from the ecu. It's also a good idea to power the relay directly from the battery, so it can cool the radiator even when the ignition is turned off.

see: http://www.keefeperformance.com/kprelay.html