You could just use 1 relay and wire then both up. 30 amps is quite a bit and one relay should be able to handle two fans. Its like over 400 watts in terms of power. I don't recommend a one relay setup though, I recommend temperature switches or computer controls to control the fans and this would mean running 2 or 3 relays.
I got 2 schematics for you. They are LT1 dual fan diagrams.
The top schematic works but having one fan turn on at low temperature at full speed. The 2nd fan can be turned on full fan speed at a higher temp.
The bottom schematic is a little better and uses three relays. It works by having both fans on at 1/2 speed at low temperature. This will make the primary fan last longer by not running it at full blast at low temp. It will also cool the radiator more evenly by having both fans on at low temp. At high temperature both fans will be on at full speed.
When the fans turn on depends on if you have temperature switches wired in, otherwise they could be wired to be on whenever you turn on a manual flip switch. Hardwiring them to a manual switch only might not be the best idea because having the fans run longer is going to shorten the fans' life.
P.S. - Summitracing sells a 10-pack package of 30A relays for $10. Too bad shipping is standard $10 but if you were buying other stuff with summit it would be a good idea to pick up some.
