I think the shading is definitely on the right track. Particularly in the one of the guy. I would watch your skull shapes though. Make sure the top part of the head above the ear at the tides is vaguely spherical. In the guy picture, it keeps veering outward, and I'm not sure whether it goes back in space enough on the other portrait. I like your features though :)

I think you should first give the priority to propotions . One more thing , that you should make a background more dark so it will not actually mix with background.

Hey there! Very nice work! Try to avoid tangents, messy lines and curves. Since your art style mainly relies on clean, straight lines, try to push that even further.

You are very well on your way! Next time you draw from reference, I think you should think in larger shapes and try to keep more of the shadow area within the same value in the beginning and then add/subtract a little bit of value from there. Try to squint at the reference image when you draw. If I squint at the male reference you uploaded the areas I circled in your drawing merges with the rest of the dark area, so I would make them darker in your drawing. If you watch Prokos YT video on shading (and many others), they tell you to keep all the shadow tones darker than all the light tones. Hope this helps, and if you have any other questions please feel free to ask :)