The events will be displayed in a single horizontal line if they don't overlap. If the end date/time of the first event and the start date/time are the same the events are still considered non-overlapping and will be displayed side-by-side in the same line.
I recommend checking the data - most likely the problem is that the events overlap.
It is also possible to force overlapping events to display in the same line using EventStackingLineHeight="0". However, it this case it would be a workaround rather than solution of the problem.