Typically in an older cabinet incubator with dual wafers wired in series such as the older GQF Sportsman 1200 series and the Leahys the second thermostat is there as a backup. The instructions for the GQF dual wafer incubators read as follows:
Your incubator is equipped with two thermostats which are
located on the side of the incubator. The one towards the front
is used to regulate the temperature desired (99-1/2o) and the
one towards the back is used as a safety to prevent damage
to the eggs should the one in front fail. Begin by installing and
adjusting the back thermostat first.
Put the wing nut on the adjusting screw. Put the adjusting
screw about three fourths of the way through the hole in the
side of the incubator for the thermostat and screw. Reach
inside and screw the wafer tightly onto the adjusting screw.
Next plug the cord into an electrical outlet Turn
the adjusting screw clockwise until it brings the wafer in contact
with the thermostat switch and you hear a light click. Notice
that the pilot light goes out when you hear this click. Now turn
the adjusting screw counter-clockwise six complete turns. The
pilot light and the heater are now turned on and the wafer has
room to expand as the incubator heats up. The red pilot light
will always be on when the heater is on and will go off when the
heater turns off. Tighten wing nut after each adjustment.
As the incubator heats up, the thermostat wafer will expand
and operate the switch turning off the heater and pilot light.
During normal operation the light and heat will cycle on and off
frequently. While the incubator heats up, watch the thermometer.
As soon as it registers 102oF (or 2oF. above the operating
temperature) turn the adjusting screw clockwise very slowly
until the pilot light goes out. If the incubator temperature does
not reach 102oF before the pilot light goes out turn adjusting
screw counter-clockwise until the temperature does reach
102oF or until the pilot light turns on.
Adjust in this manner until you have the incubator regulated
to hold at 102oF. Operate for at least 30 minutes at this
temperature to be sure it holds this temperature.
Once the back thermostat is regulated at 102oF, open the
door and install the wafer in the front thermostat. Regulate the
front thermostat for 99-1/2 oF in the same manner as for the back
thermostat.
THE CORRECT TEMPERATURE FOR MOST EGGS IS
99-1/2 oF DURING SETTING AND THIS TEMPERATURE SHOULD
BE REGULATED WITH THE FRONT THERMOSTAT. The
back thermostat is a safety to prevent excessive overheating of
the eggs (should the front thermostat fail in the heating mode)
and should not be touched once it has been set. Should it be
bothered repeat the regulating process.
Now that the incubator is regulated, slight adjustments can
be made with the front thermostat. Check the temperature
several times each day and if it is 102oF, then the front
thermostat may have failed and should be replaced. The back
thermostat can be used to bring the temperature down to
99-1/2 oF until a replacement switch can be obtained.
I copied these instructions from the GQF site and edited them slightly removing references to the electronic thermostat. Other than the location of the thermostats on the Leahy, top vs. side, these instructions should have you up and running. Just select one as primary and one as backup and go for it.
