From Dark and Darker Wiki
I believe that combining the crouch and shift walk only works as a method to decrease your movespeed. I think that whatever input you press last, whether it be shift or crouch determines the noise level of your footsteps. For example, if when trying to crouch and shift walk, you pressed crouch first, then shift last, the sound would be the same as to shift walking, just with the movespeed penalty of crouching, it could also be the other way around with your first input taking noise priority over your second, I don't really know, this is just what I think in my time playing DaD. It is important to note however that crouch+shift walking increases the amount of time between each footstep audio is played, which can be useful in its own ways to be sneaky. Please correct me if I'm wrong, this is just what I personally know from my playtime.