Right-click on the tab and click "Detach tab":
To reattach it, right-click on the tab in the floating window and click "Attach Tab".
Thank you! I never would have found that on my own!
If you have two screens you can also simply click on the tab for one of the maps and drag it to the other screen.
You can do the same thing on a single screen by resizing the MindManager window so it is no longer full screen. Then click on the tab of one of the maps and drag it off so it forms a a second window. You can then move and resize either window so they sit side-by-side.
In fact you can set up multiple windows this way, but things can get a little confusing especially with more than two windows open. This is because in MindManager's implementation of multiple windows the ribbon and quick access toolbar stay on the original window.
@Alex - agreed. Detached map windows are fine for browsing topics, but only keyboard shortcuts or context menus are practical for editing.