Hi Lennart,
It's always better to keep some species seperate. Even if they aren't cannibalistic, the prescence of another snake may stress one or both of them enough to stop eating. That being said, I have housed Atheris together without any problems before.
If you have seperate setups, it's best that you use them and only introduce before breeding. Accidents can happen if they both go for the same prey item at once or for a variety of other reasons. You have some very beautiful snakes, it would be a shame if one of them got hurt or stressed out.
Just something to think about.
All the best!