How to install a single pole humidity sensor on a 2 way switch setup
That's a common problem - integrating the functionality of a smart device with a multi-way switch.   This is often confusion over what the desired behavior is; often confusing electronics with a butler.  Consider. 


the other switch is in the downward position.  What do you want the smart device to do? 
the other switch is in the upward position.  What do you want the smart device to do? 
the smart device has turned the fan on, and you throw the other switch.  What do you want to happen?
the smart device has turned the fan OFF, and you throw the other switch.  What do you want to happen? 
if you manually turn it on, should it continue running forever?  
if you manually turn it off, obviously the smart device shouldn't immediately turn it back on.  Ok, so when should it go back to automatic operation?  


See, it gets very messy very quickly.   Of course there are obvious answers that make sense to a butler, but how do you make electronics do that?   It's not that easy.  You are better off to choose one control method and stick with it.