sometimes the attic fan is on a separate circuit. in a finished attic
with a floor it's easier to find out what wattage or amperage the fan
wants by reading its electrical plate. you could have your electrician
wire the attic exhaust fan on a (thermostat/center off/on) three
position switch on the second floor. if the fan is powerful enough it
can exhaust unwanted heat from the attic and the house if the doorway
to the attic is open, but depending on where it usually gets its makeup
air other attic inlets may need to be closed during this cycle. stand
in the attic doorway and see which way the air goes for the best ideas.
look up whole house ventilation but keep any more fans up in the attic.
motorized screened louvers may provide an intake in the attic.
see also: http://www.buildingscience.com/resources/homeowner.htm