Eventee does not allow concurrent sessions within one stage, that’s why you might come across a warning saying Your sessions are overlapping. In such a case, at least one of your scheduled sessions is overlapping with another session on the same stage.
When you go to Content -> Schedule, you can see the overlapping sessions highlighted with a red stripe:
How to fix this? Simply create a new stage and move one of the concurrent sessions to it. This way, they can run at the same time without overlapping each other in the schedule.