If you are using Windows:
(1) First, set up Windows to work with two monitors. After connecting the second monitor, you may need to right-click on the desktop background and click "Screen resolution".
(2) In Copper, click on the View menu -> Dual Screen.
(3) Drag the "Transaction Details" window to the second screen and maximize it.
You have to do steps #2 and 3 every time you restart Copper.