I drew this out for you. I embellished on the wire size scale to prove a point. The larger wires need to be large to handle the amps the fan OR fans draw. When you use (2) relays you in essence create a "current divider"--meaning when you use the same guage wires leading to and from the relays where they come together from the battery (+) and to the actual fan(s) ---they equally share the same amount of current.
In other words, instead of one relay having to deal with 30 amps all by itself, 2 equal wired relays EACH share 15 amps apiece to help each other out.
Keep the large power wires SHORT, don't be running massive wires under the dash, this:
A) is an ugly pain
B) is a dangerous fire hazard if something goes wrong.
It's okay to run the little switch wires all around and everywhere, why? because the current flowing thru them is so small compared to the fans load. Just include a switch like I showed, then the fans "won't always be on when the engine is".
Find a tap such as the "IGN" terminal in the fuse box for the wire that feeds the switch. That way YOU can only turn them on when the motor's running to prevent battery wear-down.
I'd recommend a fuse close to the battery on the main (big) power (+) lead feeding the input to the relays.
Make sure the grounds have a GOOD connection (the funky little 3 tier symbols mean ground). In fact, what you may want to do is run the big ground from the fan(s) themselves directly to the battery (-) to ensure a good connection, instead of placing burden on your vehicle's ground straps (since they link the battery negative to the car sheetmetal) if you let's say bolted it to a fender. The engine block itself is another good ground choice if it has a good ground to begin with.
The "little" grounds from the relays themselves can be mounted to any good ground, even a nearby fender bolt.
Remember the relays are made of plastic--their casings will melt if too close to an engine, radiator, or exhaust manifold.
Good luck and practice good safety!
[This message has been edited by Coppertop (edited 07-27-2000).]