To be honest, I don't think I could sleep at night knowing my nanny was leaving a newborn to watch my kids for 50 hours plus commute. I know it's a retro feeling, but it just feels so wrong and amoral.
Please try to see if you can extend her maternity leave or allow her to bring the poor little helpless baby along with her. Not ideal, but it seems so monsterous to allow the type ic situation OP is discribing.
OMG, this is ridiculous. OP, you're not your nanny's mother and it's the nanny's job to decide on the appropriate child care plan for her family. It is not monstrous to listen to and come to an agreement about maternity leave. If your nanny discovers the plan is too difficult for her, she can reopen the conversation with you and you can grant more leave.
It is a bad idea to ever let a nanny bring her own child and the many, many threads here on the subject detail well why it is a bad idea for all concerned. That said, if you are open to doing a nanny share with your nanny, make sure you negotiate share rates, check your homeowner's insurance about liability, and detail in a contract exactly how this new nanny share might work.
If you aren't open to a nanny share with your nanny, consider LWP if she says she needs more maternity leave.