first of all to fix them together with screws,screw the side panel to the door as there wont be any screws showing secondly you can buy both door with side panel in one or seperate door and side panel,if you buy a second hand door and new side panel you have to mesure thickness of door and order the same thickness side panel as it could be differant as you can get 60mm,65mm and 70mm profiles,also colours vary as well so might not match,the best way is to order new.hope this helps
shaun www.sdwupvcrepair.co.uk, September 2008