example 2

from tkinter import *

root = Tk()

def on_click(event):
    global cnt
    cnt += 1

    print('Hello!')
    if cnt == 2:
        t = event.widget
        t.unbind('<Button-1>')

cnt = 0

button1 = Button(width = 3, height = 3, bg = 'skyblue')
button1.grid(row = 0, column = 0)
button1.bind('<Button-1>', on_click)

button2 = Button(width = 3, height = 3, bg = 'orange')
button2.grid(row = 0, column = 1)
button2.bind('<Button-1>', on_click)

root.mainloop()