If a twin-engine airplane's propellers both rotate in the same direction, do they still produce a left-turning tendency?
Yes, for the most part.


Definitely P-factor will still exist and provide a yaw torque in the expected direction.  This is why twin-engined aircraft have a "critical engine".
Engine torque will still produce a roll torque in the expected direction.
Gyroscopic precession will still produce a yaw torque in the expected direction when the aircraft pitches.
The yaw torque from the spiralling slipsteam interacting with the vertical fin-- which is normally quite significant in conventional single-engine aircraft-- is the one effect that may be be greatly reduced in a twin-engine airplane of the usual configuration, since the fin is not located in a strong part of the circulation behind either prop.