I got this information off the MOM website:
To be eligible to hire a second foreign domestic worker (FDW), you have to have any of these family members living with you:
- At least 2 children below 18 years old.
- A parent or parent-in-law over 60 years old.
After fulfilling those conditions, you also need to provide sufficient privacy and sleeping space in the house for the 2 FDWs.
To apply for the second FDW, submit a Work Permit application and provide the relevant information:
- A copy of the children's birth certificates.
- A copy of the parent's or parent-in-law's identity cards and the employer's or spouse's birth certificate. Do note that each household can only hire up to 2 domestic foreign workers.
http://www.mom.gov.sg/faq/work-permit-for-fdw/how-do-i-apply-for-a-second-fdw