Effect of 0 Hz setting on VFD with continuously spinning fan
VFDs used for HVAC applications are often special models designed for that industry. You should look carefully at your manual and other documentation to determine what features are provided in your VFD. Rather than braking, it may be equipped with a feature that only allows the load to decelerate as fast as it will decelerate without regenerating more energy than is absorbed by losses in the motor and VFD. It may essentially "sleep" when the speed command is zero. It may have a feature to find the motor speed and synchronize with it if it is turned on when the motor is already turning due to air flow produced by the other drive. Various features can be enabled or disabled using drive set-up parameters. It is essential to have full documentation for the VFD.

Edit re added information:

I believe the drive is configured as for sensorless vector control. When zero speed is commanded it will energize the motor with magnetizing current, but try to drive the motor to zero speed if the shaft is turning. It will not provide DC braking unless that is configured, and I suspect that it is not configured. The drive has a braking chopper built in, but apparently no braking resistors. There will be no dynamic braking unless optional external braking resistors are connected. When deceleration is called for, the drive will decelerate at the slower of the rate called for by the deceleration ramp adjustment or the rate that prevents regeneration in excess of losses.

This drive has a group of preset adjustments that are loaded by selecting a "Macro." The PFC macro is intended for use with fan and pump loads. It sets the control method to scalar frequency control rather than vector speed control. I am pretty sure that would cause the drive to completely de-energize the motor when zero speed is called for. Since centrifugal fans and pumps produce zero or near zero flow at some speed above zero, I believe this drive has a configuration setting that would de-energize the motor if any speed below some set minimum speed is called for. I don't think this is configured by the PFC macro.

I have written technical literature under contract for ABB USA, but I have not done that for quite a while, so I am not completely familiar with this drive.