I totally agree with Cara, even I think we know each other
Notice period, same as whole contract is very important to have signed before you start the whole nanny share.
I do nanny share for 2 families where they agreed to do 1 month in one house and 1 month in the other house. It actually didn't work like that because 1 house was under building construction so we spent 3 months in one house and then were making up in the other. It worked fine. I think swapping houses has some good sides and bad sides as well. 1 of disadvantages might be for nanny who's cooking and freezing food. She needs to stock 2 freezers which means A LOT of cooking.
Also you should talk with the other family what will happen if your or their child is ill, if you'll separate them and who will have nanny and who doesn't, or how you'll sort out situation if your nanny is ill etc.
Re: live in or live out nanny
I haven't been live in before but I heard different opinions about it from other nannies. It depends on your characters how you deal with it. Sometimes live in nannies get fed up with living and working in the same place and not being able to get out and meet other people. Or they feel they need to work/do sth even after their working hours. Other live in nannies just go out and do what they want to do in their free time.
But I would never say live in nannies are more reliable than live out nannies. I definitely doesn't depend on that, but on nanny's character!
All the best x