HI Deepali,
Actually those days most of the company's or institutions were run by government, so the rule says double employment is prohibited. Even now in the public sector the double employment is prohibited. However, the same rule is being followed in private sector too. This is being followed to create job opportunity to the unemployed. But now a days nobody bothered about all these things and they are practicing privately. Even the companys are not bothered about this. Only in the govt sector, this rule is being strictly followed.
Finally as per law it is not permissible, but you can do it without the knowledge of the employer.
Regards,
Umesh.S.