in[1]:

import pandas as pd

    def somefunc(input1, input2):
        output1 = 1
        output2 = 2

        return [output1, output2]


d = {'col1': ['A1', 'B1'], 'col2': ['A2', 'B2']}
df = pd.DataFrame(data=d)

df[['col3', 'col4']] = df.apply(lambda x: somefunc(x['col1'], x['col2']),
axis=1)

print df

out[1]:

   col1 col2  col3  col4
0   A1   A2     1     2
1   B1   B2     1     2

in[2]:

import pandas as pd

    def somefunc(input1, input2):
        output1 = 1
        output2 = 2
        output3 = 3


        return [output1, output2, output3]


d = {'col1': ['A1', 'B1'], 'col2': ['A2', 'B2']}
df = pd.DataFrame(data=d)

df[['col3', 'col4', 'col5']] = df.apply(lambda x: somefunc(x['col1'], x['col2']),
axis=1)

print df

out[2]:
"ValueError: Columns must be same length as key"

KeyError: "['col3' 'col4' 'col5'] not in index"