We believe this is a single user, single stylus type application and there doesn't appear to be any way to get multiple strokes working in Sketchbook.
There's no option for the application to run as a TUIO client and it would appear this application is intended to be used by a single person working with a drawing tablet or mouse. In macOS (and we believe in Windows too) there can be only one tablet stroke occurring at a time.
In our tests we did see that Sketchbook does respond to Apple trackpad multitouch gestures (albeit with very poor responsiveness) so in theory it could be configured to allow some multi-touch functionality via gestures but not multiple strokes at once.