Use custom ajax along with this module

Hi,

I have to exposed filters : region and sub-region.
I want to hide the sub-regions until the user selects a region.
I also want to update the available sub-regions according to the selected region.

I have been able to hide the exposed filter thanks to Views Exposed Filters.
I am able to update the available sub-regions according to the selected region thanks to some custom code using the Drupal Ajax Forms API.

However, I cannot make them work both at the same time. I use the '#ajax' form attribute to make it work and I suppose Views Exposed Filters does too.
Both behavior - the hiding and the update - work the first time, but then only the hiding works and my custom update does not trigger anymore.

Any idea how I could deal with that?