Here's your suggested homework assignment:
Only play with this once you're very comfortable with our box logo, two-value system of creating light and shadow.
The power of the artist is in large part not what we add in but what we leave out.
When dealing with two light sources, you are adding something in. So ease into that complexity very carefully.
Having said that, Practice from the reference I've provided adding a secondary light to separate one new plane from the rest of the shadow.
Make sure that newly separate plane is well away from the main light side.
In this exercise, we're simply using a first and second light source to create three planes (or three groupings of like valued planes) instead of the two planes (or groupings of):
Light plane(s), shadow(s), reflected light plane(s).
Have fun, my friend!