I think you can address your requirement changing the Project Calendar. You do this using the "Change Working Time" option, available at the Tools menu.
At this dialog you need to go to the "Work Weeks" tab at the bottom. Then you select the [Default] row and press the "Details..." botton. Then, select EACH day of the week (Monday, Tuesday, ...) and click "Set day(s) to these specific working times:". In the table at the bottom, you then enter the 6-hour-per-day working schedule (for example: 8:00AM - 12:00 PM / 1:00 PM - 3:00 PM), and finally press OK twice.
At this point your scenario should work. Please try it and let us know.