```py
one = DataFrame({'a': ['apple', 'boy'], 'b': [4, 2]})
two = DataFrame({'a': ['cat', 'dog'], 'b': [5, 1]})

```

```py
>>> one
       a  b
0  apple  4
1    boy  2

>>>
>>>
>>> two
       a  b
0    cat  5
1    dog  1

```

And I want the combined dataframe (named as 'result') to be:

```
>>> result
       a  b
0    cat  5
1    boy  2

```

because 4 < 5 and result['a'][0] should be set as 'cat' instead of 'apple';
1 < 2 and result['a'][1] should be set as 'boy' instead of 'dog';

How to do that?