Netflix's help system sheds some light on streaming devices and simultaneous streaming:
Q:Can I watch movies instantly on more than one PC or Netflix ready device?
A: Some membership plans allow you to watch simultaneously on more than one personal computer or Netflix ready device at the same time. If you are on the 1 disc out at-a-time plan, you may watch only one device at a time. If you are on the 2 discs out at-a-time plan, you may watch on up to two devices at the same time. Members on the 3 disc plan can watch on up to three devices. The maximum is four devices simultaneously, and that is available for members on the 4 or greater discs out at-a-time plans.
Please keep in mind that your account can have up to six unique authorized devices activated (associated with the account) at any given time, including personal computers and Netflix ready devices. This is due to requirements from content providers. To activate a device or see your currently activated devices, visit Your Account.
For example: If you are on the 1 disc plan, you can have up to six devices associated with your account, but can only watch one of them at a time. If you are on the 2 disc plan, you can have up to six devices activated, but can only watch two of them at the same time.