Thank you Josef, it works, which is great! But how can I prevent the dark borders around my postcards?
I played around with some (un)premultiply settings and switched the shuffleUV`s alpha to one of my channels called “Front.R”, but it still remains .
After shifting the UV offset values in the STmap-node a bit I think it has more to do with my ST map overlapping than with the alpha map. There are some transition zones that on the edges that mix up the ST-coordinates.
I should probably separate my ST-map in front and back sides (and maybe even apply some bleeding which gets cut away by alpha later?).