When I create a plot with two y-axes (left and right) using ax.twinx(), draggable legends don’t work.The legend is only draggable for the 2nd y axis (right), but not for the original y axis (left).

matplotlib v3.2.0

```
%matplotlib widget
import matplotlib.pyplot as plt
import numpy as np
x = np.arange(0.1, 4, 0.5)
y = np.exp(-x)
fig, ax = plt.subplots()
ax_twin = ax.twinx()
ax.set_xlabel('X axis')
ax.set_ylabel('Left Y axis (Red)')
ax_twin.set_ylabel('Right Y axis (Blue)')
ax.plot(x, y, label='Left', color='r')
ax_twin.plot(x, y**-0.5, label='Right', color='b')
ax_legend = ax.legend()
ax_legend.set_draggable(True)
ax_twin_legend = ax_twin.legend()
ax_twin_legend.set_draggable(True)
plt.show()
```