Take a copy of your Standard calendar and add your 2-week period to this "copy of Standard" calendar using Exceptions.
Then go to your Gantt chart Timeline and change the calendar for Now-working time to "copy of Standard".
Your Tasks will have the Standard calendar while your Gantt chart will use the "copy of Standard" calendar with the only difference between the 2 calendars being the 2-week period you want to highlight.
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