I'm not sure we need to make this requirement.
Also, once you're done with the example I'd include start, stop and picks into the API and refactor code to avoid reduplication.
I would add checker for the length and the contents of the list.

``` Python
if len(raws() != 2:
   raise ValueError('...')
if not all([isinstance(r, Raw) for r in raws]):
   raise ValueError('...')

covs = []
for raw in raws:
    data = raw[picks, start:stop]
    cov_func.fit(data)
    covs.append(cov_func.covariance_ / np.trace(cov.covariance_))

self._decompose(*covs)
```