Swap 2nd thermostat for *off* swtich downstream of 1st thermostat
Just replace A with B.  In Box#1 disconnect the live wire to the switch for Fan#2, from the 120V source and connect it to the Fan#1 terminal.
If these &quot;high voltage&quot; thermostats are actually carrying the load current, not using a relay, make sure Thermostat 1 can handle the current for both fans.. Look at the labelling or documentation for the thermostat and for the fans to figure that out.