This page discusses ways to operate Armory more efficiently.
Press ctrl-alt-w to duplicate Blender window. Now you can easily move one window to second monitor. Launch player in viewport and continue editing the scene in the second window. As you work, the changes will reflect in the player.
If you are on a notebook and want to prevent spinning fans after a while, set lower value in Render - Dimensions - Frame Rate. This value can be operated while the player is running. Optionally, you can also Pause the player.