Mac OS X does not have native multi-touch support and must be added when using a touch enabled monitor with your system.
Hooking up a touch enabled monitor will only allow a single touch by default as it is mimicking a mouse.
PreSonus decided to include TUIO support which is an open source protocol for multi-touch for OS X in Studio One.
You need to enable this in the application Touch Input menu in Studio One Preferences.
Open up Studio One Options (Command + ,) or select Studio One from the Finder menu and click on Preferences.
In the Preferences menu make sure you select General, then select Touch Input Tab.
Check the box to enable TUIO support.
If you have more than one monitor be sure to enable TUIO on the numbered monitor in your setup.
Be sure to click "Apply" and "OK" to enable your changes.