Разделить или объединить действия по дате

Это было сложно, но я считаю, что это сработает для вас.

from collections import defaultdict
import pandas as pd

data = {
      'ACT1': [pd.Timestamp(year=2015, month=8, day=11),
               pd.Timestamp(year=2014, month=7, day=16),
               pd.Timestamp(year=2016, month=7, day=16)],
      'ACT2': [pd.Timestamp(year=2015, month=8, day=16),
               pd.Timestamp(year=2014, month=7, day=16),
               np.nan],
      'ACT3': [pd.Timestamp(year=2015, month=8, day=16),
               pd.Timestamp(year=2014, month=9, day=16),
               pd.Timestamp(year=2017, month=9, day=16)],
      'ACT4': [pd.Timestamp(year=2015, month=9, day=22),
               np.nan, 
               pd.Timestamp(year=2017, month=9, day=16)],
      'ACT5': [pd.Timestamp(year=2015, month=8, day=19),
               pd.Timestamp(year=2014, month=9, day=12),
               pd.Timestamp(year=2017, month=12, day=16)]}

df = pd.DataFrame(data)

# Unstack so we can create groups
unstacked = df.unstack().reset_index()

# This will keep track of our sequence data
sequences = defaultdict(list)

# Here we get our groups, e.g., 'ACT1,ACT2', etc.;
# We group by date first, then by original index (0,1,2)
for i, g in unstacked.groupby([0, 'level_1']):
    sequences[i[1]].append(','.join(g.level_0))

# How many sequences (columns) we're going to need
n_seq = len(max(sequences.values(), key=len))

# Any NaTs will always shift your data to the left,
# so to speak, so we need to right pad the rows 
for k in sequences:
    while len(sequences[k]) < n_seq:
        sequences[k].append('')

# Create column labels and make new dataframe
columns = ['Sequence{}'.format(i) for i in range(1, n_seq + 1)]
print pd.DataFrame(list(sequences.values()), columns=columns)

   Sequence1  Sequence2 Sequence3 Sequence4
0       ACT1  ACT2,ACT3      ACT5      ACT4
1  ACT1,ACT2       ACT5      ACT3          
2       ACT1  ACT3,ACT4      ACT5