As I understand it you need to have high scores in both.
The first one is used when you initially hide in shadows. That determines if you hide sucessfully. if you have a low score here you may see a "Hide in Shadows Failed* message when you try to hide.
The Move silently is used when you are moving around. When you are hidden and moving this is constanly checked to see that you remain hidden.
Hopefully I got this right as its been a while.
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