I'm using a controller (since I'm one of those weird console players).
After I first encountered the rolling issue. I tested the same Aircraft on Kerbin and no issue. This made me think, "ok, seems to be bugged". However, the interesting part was that the effect was in fact inconsistent. It is the biggest with greater angles of attack (of rotor blades) and decreases with altitude. With greater angle of attack, the rev. speed of the rotors is forcefully lowered, for both but more so in the front.
The issue didn't make the craft unusable though , it flys stable at around 12500 m (over sea level) with blade authority at 57% , front/clockwise rotor at 437rpm and rear/CCW ROTOR at 326rpm with a speed of ~110m/s. I did quite some tweaking and testing...
After all, this craft, despite or because of the issue, provided me quite some fun hours.
I just wanted to check here in the forum, if it's typical for EVE, since it's my first prop-plane or if it's bug or potential damage on the plane. I have to admit, the additional time I spent to design the plane, was saved during the creation of my decent "vehicle".
But normally, if a plane is stable and flys on Kerbin, it should also (more easily) on Eve, because of it's much denser atmosphere?
I also think, it has something to to do with the coaxial rotor design. So will test with a second craft with differently arranged rotors.