When ordering parts for my Mustang and Camaro, I got the choice of using the on-off style relay or Spal's fan controller unit.
This unit allows you to have each fan turn on at a pre-programed temperature. The first fan turns on at full power at the first selected temperature, for example 170 degrees. If the second set temperature is 190, the second fan will progressively speed up from 170 degrees to full speed at 190 degrees.
To see more about this unit, give Be Cool a ring at 800.691.2667