Running a multi-region Cassandra cluster is ill advised. Cassandra (and Dynamo databases in general) are quite chatty. I think Netflix has implemented some multi-region clusters. Other companies too I'm sure. But it will certainly give you heaps of new challenges (and heaping bandwidth bills.)
Your information is obsolete. Cassandra will only send a single copy of your updates cross-region, which will then be rereplicated within each region if necessary.
