a      b
7:05   7:45
12:40  11:04
15:04  16:40
20:50  19:49


#minutes difference between a & b
df['adatetime'] = pd.to_datetime(df['a'].astype(str))
df['bdatetime'] = pd.to_datetime(df['b'].astrype(str))
df['aminutes'] = df['adatetime'].dt.hour * 60 + df['adatetime'].dt.minute
df['bminutes'] = df['bdatetime'].dt.hour * 60 + df['bdatetime'].dt.minute
df['abdiff'] = df['aminutes'] - df['bminutes']

or:
df['asplit'] = df['a'].str.split(':')
df['bsplit'] = df['b'].str.split(':')
df['aminutes'] = df['aminutes'].apply(lambda x: int(x[0])*60 + int(x[1]))
df['bminutes'] = df['bminutes'].apply(lambda x: int(x[0])*60 + int(x[1]))
df['abdiff'] = df['aminutes'] - df['bminutes']