Hi jjthor,
I can give you some instructions on what you need to do to login to MLB.TV. You would go through the following steps:
1. Visit https://secure.mlb.com/tv/activate/ and select Cox Communications as the provider.
2. Login using your Cox credentials.
3. Link the Cox credentials with the new or existing MLB.com account.
I read through the information on our site and the MLB site, what I gather is that you are only allowed to stream one device at a time. There was no information to suggest that you would be able to stream on multiple devices. I hope this helps, let us know if we can assist you any further.