By default users are not able to select units who's times overlap. However you are able to over ride this altogether or select the number of minutes time units are allowed to overlap. To begin, make sure you are in the Scheduler interface by selecting the “Schedule” under the "Settings" menu.
Next, click on “Actions” and select “General Settings.
Turn “Disable time conflict management” “On” to allow users to select time units regardless of if they over lap.
Turn “Enable time conflict allowance” “On” and enter the number of minutes that are allowed to overlap with time units.