Hi Olvisab,
What do you mean by parallel to the ceiling? If you mean installed and connected to the ceiling I do not think this is a good decision personally. It would be far too high and while you could theoretically angle the speakers downwards to a middle back sweet spot if you will, I still don't think you'd be getting the best from the speakers.
Having said that, it would still be OK since they are rears, so to a large extent not as significant as the fronts, but really you want to have them just above head height to the side of you, even though I acknowledge you say this is not easy or even possible. Could you not still use floor stands and have them next to the couch on either side? They do not have to be behind the couch necessarily, just to both sides. Simon.