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 switch monitor icon.
Update, version 22.214.171.124 and newer for streamer and Business app (www.splashtop.com/downloads):
Then choose the monitor to control or the bottom option to show all monitors (Show All Monitors only available on Business Access, Remote Support Plus, and On-Demand Support services - not available on Remote Support Basic):
Version 126.96.36.199 and older:
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 remote 2 monitors show on your local 2 monitors at the same time.