The easiest way to switch between these applications (in fact, between any Windows applications) is to press <Alt+Tab>. Pressing this combination once takes you to the previously active window, which will often be the other STEM window, when you are working on a model.
More generally, pressing <Tab> while holding down the <Alt> key steps through a set of icons, representing each of the applications currently running on your PC. Release the <Alt> key to jump to the selected application.