You need to set the axis to 1.
pd.concat([df1, df2], join='outer', axis=1) A BDateTime 2017-04-19 11:00:00 3.0 NaN2017-04-19 12:00:00 5.0 13.02017-04-19 13:00:00 1.0 15.02017-04-19 14:00:00 7.0 1.02017-04-19 15:00:00 NaN 17.0