It's probably best to face the two challenges in the order listed, so that you and the other sharers can hire the nanny together and be sure everyone is fully on board with that crucial choice. There are a lot of issues to negotiate in setting up the sharing arrangement, too--hours, transportation, what the children are to be fed, what types of activities you want them to be engaged in, and how you'll share costs and deal with taxes. All of these questions are resolvable, especially if you talk about them in advance--as with most sharing agreements, communication is key here.
Sharing child care has a ton of advantages. It saves money for everyone, conserves resources, and provides children with opportunities to socialize with others in a home environment. Challenging as it may be to find the right setup, once you do it's likely to benefit your family in the short and long term.