DataFrame.plot returns/accepts one axis when using secondary_y
#### Code Sample

```python
z = pandas.DataFrame(numpy.random.rand(10,2), columns=['a','b'])
axes1 = z.plot(secondary_y='a')
axes2 = axes1.get_figure().get_axes()
z.plot(secondary_y='b', ax=axes2)
```

#### Problem description
```axes1``` is the left axis, instead of both the left and right axes (like ```axes2```). Also, the last line results in
```python
AttributeError: 'list' object has no attribute 'get_figure'
```

#### Expected Output
When ```secondary_y``` is not ```False```:
* ```axes1``` should be a list of both axes.
* ```DataFrame.plot()``` should accept ```axes2```.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0
pytest: 3.5.1
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.2
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
