It is normal that both lights come on as you say. Unfortunately where the problem lies is with fitting a HID kit to a standard headlamp/reflector assembly. 'Type approval' and 'homologation' are required for every part of a car to allow it to be used on the roads, however these two approval mechanisms work on the complete unit only. You cannot type approve or homogate just a HID or a Headlamp unit in isolation. Therefore if you fit an HID unit to your car you are to all intensive purposes breaking the law.
HOWEVER, I worked in the headlight industry for over 10 years and never heard of anyone getting done for this, and from a practical point of view just ensure that the centre of the HID bulb lies at the same point as the centre of the Halogen bulb. Basically park your car up against a wall shining the headlamps onto the wall. You will see the beam starts at about 30 degrees coming down to the axis of the light (in other words straight in front of the headlight), and then runs horizontal to the right, the height of the beam should be at the same height as the bulb relative to the road (with levelling at 0). The beam itself should have a VERY crisp line on the wall, below the beam bright, above the line dim.
If the line between light and dark is not crisp and is mushy, or the shape isn't as described above then you need to see if the HID buld is in the right place. If you can't set it right then put halogens back in because the police will do you for that. I myself modified my Golf headlights and was stopped one night and had to shine my lights against the wall. Luckily I had made some spacers to put the HID in the right position (however the Golf headlight was never designed for HID so they were as bright as daylight but the police can't check this.)
Hope this helps.