Actually, you can't. It will just maximize the first WE. Blizzard made that to prevent multibugging and testing multiple maps - Warcraft can't be multi-opened, too.
You can have multiple maps opened at once, as stated before, but there is a bit of a transition period between switching back and forth between open maps.
When you have map A opened, open map B. To switch, click on the <Window> tab and click the map you want to jump to.
I explained how to do it, you did not.