i would imagine there is a seal between those otherwise condensation would set in.
you need to take unit out and somehow try and heat the seal to get it apart.
There is a loose rubber seal between the two halves. as i have seen a quote somewhere (i wish i could remember where! ) that both lenses could be changed in a maximum of 30 minutes, then i'm pretty sure that the headlight units weren't removed from the car. thanks anyway.