Dual monitors and flash movie to stay maximized on one of them

It actually is possible to do! You do have to either hex edit a flash binary or you can use the one he supplied if you trust it.

"Watch fullscreen flash while working on another screen"

No, you can't do that with flash, but for the TWiT Live stream, you can use VLC to play the following stream: http://bglive-a.bitgravity.com/twit/live/high and that will allow you to play it on a second monitor in full screen.

It's currently not possible. According to a reply to a bug report on Adobe:

We understand that many users would like fullscreen on one monitor and to be able to interact with your OS on another monitor. However, due to security requirements, we require that Flash and Browser must be the current focus of your OS. Apologies if this causes any workflow or usability issues. We will continue to review our security policy and consider changes in the future.

Some sites let you watch "pop out" screens. They stay maximized so you can work on the second monitor. An example of this is www.hulu.com - on the right side bar of the screen there is a menu - choose "pop out and a new screen opens up. I suggest you check the FAQ or search the knowledge base of the video sharing site and find out if they offer "pop out" screens.