Comment 1 for bug 1339710

Multi-touch in Onboard is supposed to work as you described, Andreas. I heard of people successfully using it in the wild too.

Please check if
Preferences->Keyboard->Advanced->Touch input
is set to multi-touch. This is the default.

Check if you own a Wacom touch-screen. Those are by default configured to not allow multi-touch, see also bug #1297692.
$ xsetwacom --list devices

If this returns your touch screen you'll have to disable Wacom gestures in order to get multi-touch.
$ xsetwacom --set <your touch-screen id> Gesture off

You can use the output of the "xsetwacom --list devices" command as id, or pick it from
$ xinput