There are 2 different interpretations of the term:
(a) 2 video streams of different frame rates. For example, one is 8fps for recording and one is 1fps for remote viewing over the Internet. The Internet connection has a limited bandwidth and requires the video stream to have a lower frame rate.
(b) 2 video streams produced by different compression techniques. For example, one is MPEG4 for recording and one is MJPEG for viewing on a mobile phone. There are compatibility issues in addition to the compression technique but dual streaming is needed if mobile viewing and local recording are needed in parallel.
Not all cameras have dual streaming. Most have type (a), and some have type (b) in addition to (a).
Update on 16 August 2011
We have tested CAK-7211 which is a new camera model with 4MP and supports Dual Streaming with NVR Enterprise version 2.3. We have observed a pixel resolution limit of 1280 x 720 under Dual Streaming. The 1280x720 limit applies automatically when changing to dual streams even if it is set to 4MP single stream. After changing back to single stream the resolution will not automatically go back to 4MP and will stay as 1280x720.