The next TeeChart for .NET maintenance release will include a new Gantt MultipleNextTasks property.

 New MultipleNextTasks for TeeChart .NET Charting LibraryThe AddMultipleNextTasks method enables more than one next task line to be added to any given Gantt bar.
This chart image shows how this looks like; the first bar (blue) has two next tasks (next task pen in Goldenrod color).

February 01, 2012