Joined in middle

According to section 192, it is the employee who files Form No 12B.

If Form 12B is filed, then current employer can deduct the TDS on salary paid by previous employer (in case no TDS was deducted by previous employer). And if the TDS was deducted by previous employer, any excess or shortfall can also be adjusted.

It is always in the interest of an employee to furnish such details because otherwise there can be duplication of exemptions and deductions and there can be a shortfall in tax deduction and as a result the employee would become liable to deposit advance tax.