If x1 and x2 are 2 individual series and of the equal indexes ,you can use series.dt accessor to parse dt.date from x1 and dt.time from x2 and bind to x1 after converting to string, then convert it back to datetime:
pd.to_datetime(x1.dt.date.astype(str) +' '+x2.dt.time.astype(str))
0 2018-04-25 00:53:00
dtype: datetime64[ns]
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