I updated the wiring in Nov 2009 to use 2 Engine Temp Switches.
One is 185 on/off This controls one fan. The other is 185/170 on/off for the second fan. I had a
200/180 for the second fan which never seemed to work well. After 18
months tests revealed it was not functioning to temp spec. I replaced
it with a 185/170.
I like the mechanical switches. Simple and redundant back up. Both temp sensors would have to fail for both fans to fail.
Note on temperature switches and designed location. Getting a switch to work correctly for a fan when installed at the thermostat or water neck area is key. Many switches are designed to be put in the head. A 195 rated switch designed for the head may turn on at 215. Head location for a sensor is higher temp than what water temp is at a thermostat. Many of the "head" based sensors may be adjusted to account for the difference. Rated 195 at the thermostat may be 215 or so at the head/sensor location. Putting sensors designed for a head in the thermostat or water neck area may lead to excessive heat before they activate.
Why not use a new electronic control? I looked at all the high tech fan controllers that could be adjusted and control 2 fans. The problem I had was they were cheap or sensitive to high voltage. The better ones have to be in a safe environment, inside the car and sensor wires can not cross Ignition wires etc. Maybe one day, but not for now.