I would set up the two shifts using two separate calendars. I would set a "day" (File > Options) as 12 hours. The 7a to 7p should be fairly easy to create. The calendar that spans over night will have to roll over to the next day. For example: the task starts Monday at 7:00 pm. So the 7:00 pm to 12:00 am on Monday. Tuesday will start at 12:00 am to 7:00 am. Then starts up again Tuesday night at 7:00 pm until 12:00 am.

You'll want to set up your resources using one of the base calendars. Then as you assign those resources, the start of the task will adjust based upon the working calendar.