Using a second thermostat to balance temperatures thru fan-only control
In theory this should be easy -- have the second thermostat connect the R and G wires when it calls for cooling, and the fan will run.

Here's one reason it might NOT work, and it's subtle/non-intuitive so I thought it was worth mentioning:  Some thermostats short the Y (cooling) and G (fan) wires together whenever they're in cooling mode.  They do this so that they can use one relay to turn on both fan and A/C during a call for cooling.  (Relays are relatively expensive components.)  This shorting happens whenever the thermostat is in "cool" mode, whether it's actually calling for cooling or not.  (Although if you set such a thermostat's Fan switch to "ON" instead of "AUTO", the short is removed so as to run the fan only.)

Where there's only one thermostat in the system, this shorting behavior does no harm.  But if a second thermostat is added and it calls for "fan", the power will go through the short in the first thermostat and turn on the A/C as well.  I have this exact arrangement in the house I just bought, and I cannot run the fan independently from either thermostat -- it will always run the A/C as well.  If I unwire either thermostat (or set it to "heat" mode), I can then run the fan without the A/C.

So you might try shorting R (or Rc) and G with your existing setup to make sure that the A/C does not come on as well.

Source: personal observation confirmed by isolated testing on Robertshaw 9600 programmable thermostat