Fix Python – Secondary axis with twinx(): how to add to legend?

I have a plot with two y-axes, using twinx(). I also give labels to the lines, and want to show them with legend(), but I only succeed to get the labels of one axis in the legend:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc(‘mathtext’, default=’regular’)

fig = plt.figure()
ax = fig.add_subplot(111)