List with Multiple Approvers

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I would like to use the calendar function of Sharepoint Team Services
however I would like to modify so that if a user inputs a date, it will
not be approved (or entered into the calender) until his/her manager
approves the date. The problem with STS is that only one person is the
approver whereas under this scenario, I would like an email to be sent
to his/her manager who then approves the date.

Does anyone know if this is possible? Having this feature in STS would
greatly expand it's functionality.

Thanks.

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