Merge on single level of MultiIndex
I don't think we can do this now, but it potentially sounds useful...

http://stackoverflow.com/questions/16650945/merge-on-single-level-of-multiindex
https://groups.google.com/forum/#!topic/pydata/LBSFq6of8ao

Example:

```
In [10]: df = pd.DataFrame(randn(10,5))

In [11]: df1 = df[range(0, 3)].set_index([0, 2])

In [12]: df2 = df[range(2, 5)].set_index([2, 4])
```

join/merge on 2.
