I would like to know this too.
It would be really cool if Flex could do it.
You can tear away the tabs and just have the source code tab on the main monitor.
Move the outline, flex navigator console and problem tabs etc..etc..to different monitors.
I haven't been able to figure out how to have source code view and design mode view open at the same time though.
Create different perspectives, and then use ctrl + ~ to quickly switch between perspectives.