Your second approach is quite close to what I think you want. You just need to store the original x-limits and then use those to reset the plot after it's resized:

import matplotlib.pyplot as plt

x1 = [3, 4]
y1 = [5, 8]
x2 = [2, 5]
y2 = [4, 9]

fig, ax = plt.subplots()
ax.plot(x1, y1)
xlim = ax.get_xlim()
ax.plot(x2, y2)
ax.set_xlim(xlim)
plt.show()
plt.close()