Finding a nanny is hard enough. There are sites like Craiglist, Care, etc. Where does one go to find other parents in a region who might be interested in a nanny share?
The only way we've ever found things like that, for babysitting/carpooling/nanny shares, is in our organic network. Check with your neighbors (if you have a bunch of people around you who have older kids, or grandkids, they might know someone who knows someone...). Talk to people you socialize with or work with. Have your friends check with THEIR friends. Post a few things on Facebook. If you participate in a religious community, ask around there. It obviously depends on where you live and the population density, but I can guarantee you're not the parents who'd be interested in something like this.
In our area there would be a few options:
Neighborhood listservs: many of the local neighborhoods have listservs, and even if yours doesn't you could see if nearby neighborhoods do and if you could get a post there.
Local message board: we have a fairly active nanny section of a local message board and there is a section there for nanny shares.
Word of mouth: as stated above, get the word out to all of your friends to check with their friends, ask around at your religious community and others of the same religion/denomination in your area, post on social networks to your friends, and so on.
Ask the nanny: maybe someone else has contacted your nanny candidate who she thinks would be a good match.