Ask the new joinees to give you the Salary slips received from previous employer and Form 16 or else take a declaration from them that they will re-work their Salary working considering Salary received from both the employers.
If the new joinees are not willing to provide you the necessary details then consider the salary received from current employer in calculation of TDS. Dont consider deductions under sections 80C, 80D etc. This is because they might claim benefit of that much amount from the previous employer as well as from the current employer.