It could be a issue specific to that game. You might check to see if there may be any updates or patches for the game at it's website available to download and install that may address this.
When both are open, check in Task Manager (CTRL+SHIFT+ESC) to see if they are both using the same process. If they are different processes, then there is nothing you can do.
- Under the Applications tab, right click on each listed opened window for it and click on Go To Process.