Multi-user drawing is not something that is on our roadmap (the map interprets multiple touchpoints as zooming/pinching).
However, it is fairly simple to do something like this using a little bit of custom code. Use the Touch.FrameReported event to detect all touchpoints, and the Map.ScreenToMap on each point to build a polyline for each touch id.
To prevent the map from navigating while you do this set IsEnabled or IsHitTestVisible to false.