Rows missing after concat two dataframes

Several rows seem to be missing after `concat`, reproducible example: 

```
X = pd.DataFrame(dict(a=list('aabbcc')))
X = dd.from_pandas(X, npartitions=2)
y = pd.DataFrame(dict(labels=list('010011')))
y = dd.from_pandas(y, npartitions=2)
dd.multi.concat([X, y], axis=1).compute() # this should give 6 rows instead. 
```

Anything important to note that I missed? 
