I have done a nanny share, although it was a while ago.
The problems I had were that one family wanted us to be more home based, I.e, not go out more than three times a week, and then only a morning or afternoon, not the whole day so no trips to the zoo or beach or even a picnic in the woods etc. the othe family had an older child who had classes every day, the former family never took it up with latter family and just tried to get me to change our routine. The latter family "found" the former family and they should have discussed their priorities before starting the share.
The money for that share was 1/3 for one child family and 2/3 for two child family.
Holidays were a real problem, it stated in my contract that each family choose a week, a week at Christmas and I choose two weeks but I very rarely got my choice of weeks because the families would book holidays, I'd still have to work for the other family and then when it came time for me to have a holiday, they had no holiday left to cover me. Which, in essence, wasn't really my problem but they would moan about having to pay double for a temp nanny, or having to take unpaid leave and so made it hard to say "no, I'm going."
The first year, I had two days off apart from bank holidays and christmas week.
The following year I booked up a weeks holiday and two odd days right at the beginning of the year but that was all I got and I couldn't have the odd day off, for a distant wedding/party or visit to friends because again they had used all of their holiday allowance.
Another problem was food, one family was really laid back about what they ate, the other family wanted a more specific diet and no snacks unless fresh fruit. The tantrums we had when one had an ice cream or biscuit or some crisps were terrible, but as the no snacks family joined after I had been there for three years (in another share but with the children's cousins and no problems about anything) it was hard saying to the three year old, no you can't have xx or yy.
Also, the new family were terrible time keepers, I had two dental appointments in the 18 months I worked for both families, the plan was for a parent from each family coming for the child over lunch time (they worked locally, I specifically booked for lunch time) new family turned up 40 minutes late and then just sat in the sitting room on his phone while old family looked after all three. They were almost always 15 minutes late home, meaning I had to stay late with old family or, if I had plans, leave child with old family, that got old real quick but the only option would have been to leave child o. The doorstep!!
There were more problems including late wages, no spares (nappies, wipes, clothes, etc) forgetting to leave car seats, not sharing their toys.
We did one week in each house and I spent most of Monday popping round to the other house for high chairs, prams, travel cots but I liked doing one week in each and it's good for the kids to spend time in thir own home.