Hi Marco,
Have you got the bicopter flying OK in rate mode, using only the gyroscope for flight stabilisation?
Have you tried flying the bicopter with only the proportional (P) part of the PID loop, (with I and D turned off)?
Most multi-rotor aircraft work on the same principle, the main difference just being how the raw throttle plus roll, pitch and yaw PID outputs are divided amongst the motors and servos. The percentage that each motor receives is calculated using simple trigonometry, for example sin(60 degrees) = .866 = 87%).