t1 = [['Valenciennes', 24],['Lille', 23],['Laon',
    31],['Arras', 18]]
t2 = [['Lille', 62],['Arras', 53],['Valenciennes',
    67],['Laon', 48]]

for i in range(len(t1)):
  for v in t2:
    if v[0] == t1[i][0]:
      t1[i].append(v[1])