[Radek]

Ultimum refugium: AFAIK, both keys are identical. Try hacking the saved game. Make a special saved game for the hacking. Place in the inventory an item which occupies a single "square" in it and then rewrite the item with the remaining key. Then go to the Cet's chamber and try to unlock both locks by your keys. If you succeed then you have solved your problem. You can then decide, whether you want to go to Cet right now or later.

Note: Read the key data first. Somewhere in the data item must be a location of the item in the inventory. Search "coordinates" in the squares in the data. You will need to fix the coordinates at the hacked key.
Take the remaining key and place it in some known coordinates and find the corresponding data. Move the key to several other locations and check whether you are right. Then hack the file and fix the coordinates of the hacked key.