dup.py

import pandas as pd
import numpy as np

df1 = pd.DataFrame([[1,np.nan]], index=[1], columns=["a","b"])
df2 = pd.DataFrame([[np.nan, 2]], ndex=[1], columns=["a","b"]) 
df3 = df1.append(df2)
#     a    b
# 1  1.0  NaN
# 1  NaN  2.0

# Combine rows with like indices
df4 = df3.groupby(df3.index).first()                                                                                                                                

# 
#     a    b
# 1  1.0  2.0

# This might be faster for larger DFs?

dup = df.loc[df.index.duplicated(keep=False)]
dup = dup.groupby(dup.index).first()
df5 = df3[~df3.index.duplicated()]
df5.loc[dup.index] = dup

#     a    b
# 1  1.0  2.0