To assign multiple shifts to the same employee on the same day, you have to enable 'departments'. This is done by selecting "Options > Show Info Column" in the menu.

After selecting this option, you'll see an additional column:

Click on a cell to add your department info, enter the text and press enter (or click somewhere else to save)

To add a new "shift line" for this employee, right click on the "infocol" and choose "Add shift line":

Now you can assign two shifts for this employee on the same day. If you need to add more departments, simply repeat this procedure.

To remove a shift line, simply right click on the infocol and choose "Delete Shift Line".
To remove the InfoCol, make sure each employee is only listed once (thus without departments), next click on the menu Option > Remove InfoCol