Both of the cars will start if the gas heater is off.
They won't start if the heater is on.
The garage is very damp when heater is on melting the ice.
Are Legacies susceptible to damp?
Dampness does impact the density of air that MAF reads. So there might be a connection there. When the heater is on and you try starting the next day, is after opening the garage door? If not, try starting the car without opening the garage door...