Some customers have asked about using a device as a second monitor instead of mirroring. This is not a supported usage of Splashtop Personal remote access product. You can find articles/videos by users on the internet if you want to try on your own.
Additionally, a friendly user created a forum to explain their steps:
For supported extended monitor Splashtop applications, please take a look at "Splashtop Wired Xdisplay" in the iOS app store or Google Play store.
Apple store: https://itunes.apple.com/us/app/splashtop-wired-xdisplay-extend-mirror/id1029826922?ls=1&mt=8
Google Play: https://play.google.com/store/apps/details?id=com.splashtop.xdisplay.wired.pro