In PlanningPME, if you try to create 2 tasks at the same time to a resource, you get the following message :
If you need to assign several tasks at the same time to a resource you can activate an option to assign several tasks at the same time. To do so, go to the menu Tools -> Options and untick the option "assign only one task at the same time to one resource" as shown on the picture below:
For more information, have a look at the documentation