Was able to find it: not a double rifle but a Doppelbüchsdrilling with two barrels in .35 Win over a 20/70 shotgun barrel.
Its a first class gun from Duchateau /Liege as we can see and was or allways is in the possession of Günter Freres if he is still alive(don't know it). He was working for DWM until the end and is a real expert for DWM ammunition.
you see that FN in Belgium was making the 35 Win after WW 2 for the local market because there was and maybe is lot of rifle's around. BTW I have heard once that all rifle ammo with FN headstamp was made from Hirtenberger. maybe someone can clear this up.