Examples

import numpy as np
import pandas as pd

df1 = pd.DataFrame({'X': [None, 0], 'Y': [None, 5]})
df2 = pd.DataFrame({'X': [3, 3], 'Y': [4, 4]})
df1.combine_first(df2)

	X	Y
0	3.0	4.0
1	0.0	5.0

Null values still persist if the location of that null value does not exist in other

df1 = pd.DataFrame({'X': [None, 0], 'Y': [5, None]})
df2 = pd.DataFrame({'Y': [4, 4], 'C': [2, 2]}, index=[1, 2])
df1.combine_first(df2)

	C	X	Y
0	NaN	NaN	5.0
1	2.0	0.0	4.0
2	2.0	NaN	4.0