If you have two pandas DataFrames you want to join where the index in both DataFrames are and you want to obtain a DataFrame where the respective columns are set to NaN if there is no value from the respective DataFrame, this is typically the correct way to do it:
df_compare = pd.concat([df1, df2], axis=1, join='outer')
If you only want to keep values where both DataFrames have some value, use join='outer'