[Tkinter] DUAL WINDOWS...

[DT2000]

You did not define the Label nor pack or place the Label widget which needs to be done for it to be published to the screen.
Here it is with the font size changed to 12 so it is fits within the windows.

from tkinter import *

window_left = Tk()
window_left.geometry("300x300")
window_left.configure(bg='black')
window_left.title("LEFT")

window_right = Toplevel()
window_right.geometry("300x300")
window_right.configure(bg='black')
window_right.title("RIGHT")

win_left = Label(window_left, text="HELLO WORLD FROM LEFT", font=("Droid Sans Fallback", 12), fg="White", bg="black")
win_left.pack()

win_right = Label(window_right, text="HELLO WORLD FROM RIGHT", font=("Droid Sans Fallback", 12), fg="White", bg="black")
win_right.pack()

window_right.mainloop()
window_left.mainloop()