When you connect to a computer with Splashtop, you will connect and see the main monitor screen. To change to view/control a different monitor, you click on the downward arrow icon at the top center of the window () to open the menu and click on the Change monitor icon.
Each time you click on the icon, it will rotate to view another monitor in a "round robin" fashion. Enhancements are in progress to allow you to choose the specific monitor to view.
For dual monitor to dual monitor configurations, development is in progress for the ability to have 2 monitors show on your local 2 monitors at the same time.