The attached test program produces no errors, while doesn't provide serial consistency. There is no exception that the consistency level is incorrect. It's simply silently downgraded. The issue is not documented either.
kostja@atlas ~ % python test.py
The behavior is contrary to the original intent by LWT author, since the code has the following check:
if (group.queries.size() > 1)
throw new InvalidRequestException("SERIAL/LOCAL_SERIAL consistency may only be requested for one partition at a time");