At either end of the backs there is a 'lock' of sorts - once you've folded them down you'll be able to see them, push a slothead screwdriver against the slot which should be obvious and it'll roll away from the post which extends from the seat into the bracket. Once you've got that edge out it's just a case of manipulating the seat back until it comes away from the bracket towards the centre of the car (there's no unclipping or similar required for this one - just brute force and ignorance)...
The job is easier if you take one of the two parts out first - I'd guess it's the 1/3 but I think that's probably too obvious so it may well be the 2/3 portion... Sorry my recollection isn't clearer!
Regards,
Rob.