How to wire garage exhaust fan to humidity sensor/fan control and ventilation controller
While it's OK to have switches in parallel...

While putting switching devices in parallel is not a common way of wiring a house, there is nothing in Code that forbids it, and it is done in other cases as well (such as multi-location occupancy sensor control).  However, such a configuration comes with caveats; most importantly, you'll need something to serve as a disconnecting means so that you can manually turn this fan off in case it needs servicing.  If you don't care about what else is on this circuit, then the circuit breaker will serve the purpose, though.

... you need to be careful when you do so

Even with the issue of a disconnecting means addressed, you still need to be a bit careful with switches in parallel, to avoid inadvertently creating a looped or paralleled neutral situation, which can be read as violating NEC 300.3(B)/310.10(H).  

How to wire this

As a result, I would wire this by using a two-gang box at the power source location with the vent controller taking up one of the gangs, then running a new-style (neutral included) switch loop to where the humidity sensor goes and wiring that switch loop in parallel with the vent controller; this can easily be done by using black for always-hot, white for neutral, and red for switched-hot at each location, then connecting matching colors together.  You can then run switched-hot and neutral to the fan from whichever box is more convenient; just tape the switched-hot to the fan from black to red to make the colors match nicely.  This does rely on the breaker as the maintenance disconnect for the circuit, but provides neutral at both locations, as is required by the fan controller and humidity sensor you specified, and means that either device can command the fan on.