I am not an employment attorney. I am willing to share my opinion however....
In a nanny share where the nanny works for both employers at the same time (this is a key point, definitions of nanny share abound!) so long as the nanny's total hourly compensation meets minimum wage and overtime rules, how this is divided among each employer is not important.
In a practical sense, I rarely see either family's contribution fall below minimum wage; however there are for instances. Two families with 3 children, one with 2 and the other with 3, may choose to pay a nanny $18/hour. The family with one child may pay $6/hour and the family with 2 $12. Total hourly compensation to the employee is $18.
Here are things I would suggest to protect everyone:
1. This has to be guaranteed compensation. Meaning the employee is paid by the two employers the total rate no matter if one family is missing one or more days.
2. Overtime has to be considered. If $18/hour is a blended rate for 45 hours for instance, you need to back it out to hourly and overtime. In this illustration the hourly wage being offered is $17.05. The overtime wage is $25.58. (our Hourly Rate Calculator will do the math
https://www.4nannytaxes.com/calculator/hourly.cfm)
3. I strongly encourage a 3 way work agreement between all parties that spells this out. (A best practice anyway, but especially if you want to make the case for joint employment for FLSA compliance purposes.)
4. Recordkeeing is important - time and attendance, calculation of gross wages, calculation of deductions and documentation of net pay.
I hope this helps. I am unaware of any wage and hour complaints in a nanny share where the considerations above were documented and there was a ruling against one or both of the employers.