I have just installed a DeWitt radiator into my 78 with a 350 engine. The radiator has 2 fans on it. My question is should i wire one of the fans to stay on with engine running and the other to the thermostat on radiator?

If the engine is below the thermostat temp why would you need a fan running?  I think I would hook them both up to the temp switch.

I'm very satisfied with my DeWitts radiator and I don't have the electric fans.

I agree with Gary that there is no need to have a fan running constantly.  When the engine temp is at or below the thermostat temp the thermostat is closed so there isn't any coolant movement through the radiator.  The fan isn't needed in that situation.

From what I have come across one fan is typically controlled by the engine temperature so would come on and off based on that.  The second fan is triggered to come on either when the engine temperature hits a higher temp or when the a/c is turned on.

My 2 pennies worth.

Just note that if you have AC you need to have at least one fan on whenever the AC is on

If not the pump pressure will raise and cause the compressor to stop spinning.

I installed a second relay to turn on with the compressor 

In my experience, hooking up the second fan to come on when the AC compressor engages isn’t ideal  

While I don’t have DeWitts radiator, I do have electric fans in my 77. It’s a dual speed single fan from a Taurus. Low speed on my car was triggered by the temp sensor. High speed was triggered by the AC compressor.

What I found was that if the low speed was keeping up with engine temp, the high speed coming on with AC would drop engine temp drastically - like 30 degrees.

Conversely, if the low speed was NOT keeping up, the only way to get more cooling was to turn on the AC. 

I didn’t want that, so I’m making the change to a dual range temp sensor. If the low speed can’t keep up with the engine heat, the high speed will come on when temps hit around 210. No need to have it hooked into the AC circuit in that case. 

In your case, the even though you have two fans I think the situations are similar. Go for tying them into engine temp, not the AC. 

Can you please tell me how you connected your additional relay to run on dual Dewitt’s radiator by identifying PIN numbers on relay as they are connected? I’m trying to get AC to spin when needed but have no luck. Both fans work but I can’t figure out how to get it to start for AC to run. Please help thanks! 
I have a 1978 corvette auto all stock except for new radiator 

I’m talking about the second relay you installed. Can you share how the second relay is connected and what relay pins they are connected to. Pics would help also 

thanks? 1978 Corvette auto 

Here’s my wiring diagram. A couple notes:

1. The pink wire is my 12v source for the relay trigger, and it’s switched by the ignition. It is NOT the main feed for the fans! You will blow the fuse if you try to run the fan off it because it’s only designed to provide 20a.
2. I have added three switches in my AC system- a thermo switch that shuts off the compressor if the evaporator gets too cold (prevents ice up), a high pressure cutoff switch that shuts down the compressor if system pressure gets too high. These are using power from the brown wire leading from the HVAC controls on the car. The last switch Turns on the hi speed fan if system pressure gets too high.

Hope this all makes sense. Let me know if you have any questions.

I should also mention that the red and blue wires that directly feed the fan (relay lugs labeled 30, 87, and 87a) are all 10AWG wire. They have to be heavier than the control wires because my fans draw a lot of amperage. 

How does it cause the compressor pressure to rise?  The fans are to cool the engine via the radiator, not the compressor.