concatenation

df = pd.read_csv('https://vk.com/s/v1/doc/Q8dE2aqabEx6LLW_FVGZLwrYk0afhuc1JKCC5PfvrlxBgaKyrGI',
                 names=['date', 'feature_1', 'price_1'])

df['feature_2'], df['price_2']  = 'сайт2', 111111

columns1 = list(df.filter(like='_1', axis=1).columns)
columns2 = list(df.filter(like='_2', axis=1).columns)

# можно так:
# df['merge_1'], df['merge_2'] = '', ''
# for column in columns1:
#     df['merge_1'] += df[column].astype(str)
# for column in columns2:
#     df['merge_2'] += df[column].astype(str)

# или так (чтобы понять суть):
df['merge_1'] = '1) ' + df['feature_1'] + '; 2) ' + df['price_1'].astype(str)
df