I had a nanny share position a few years ago now, they lived a street away from each other, there was 5 weeks between the babies, 3 and 4 months. I did one week at one family home and the following week at the other families home, I didn't do any nursery duties but that was just the way they wanted it, if you swap houses each week you could get the nanny to do nursery duties for you.
They brought a car between them, it wasn't top of the range but it was a safe car and we transfered car seats at the beginning of the week and back on Friday night. They brought a travel high-chair between them and a travel cot and they were brought over on Monday morning with the baby.
Holidays - In a "normal" nanny job, the nanny chooses 2 weeks and the employer chooses 2 weeks, in my nanny share, I had a week at Christmas and then we each choose a week. It worked well, if the families wanted 2 weeks holiday we all sat down and tried to find two weeks we could all do or they just took extra holiday (tbh most of my employers have had much more than 4 weeks holiday entitlement a year!)
If a child was sick we generally carried on as normal, the children spent 10 hours a day with each other so if one got a bug the other one will get it! They usually are contagious a few days before the symptoms show anyway. Or they pick it up from other children we spent time with. I even looked after them both when they had chicken pox (stayed in for almost 3 weeks!!)
The only problem we had was when both families expanded within 4 months of each other, one family (who found one child hard work) thought that it would be too difficult for me to look after 4 under 23 months! They found a new nanny without telling any of us until the friday before the new nanny started, I threatened to take them to court to get my 4 weeks notice money, then they found out that the nanny they took on had a back problem (not perfect for someone who had to look after a 22 month old and 7 month old!) So they got rid of her and came crawling back, we hadn't found another family to share with so we went back to original arrangement but there was too much resentment and worry they would do the same thing again so it ended after 6 weeks, I was gutted cos I had so much fun with all 4, the older 2 were so close and it ended badly so they didn't see each other for 7 years.
The third family we did the share with did not like the fact that the then 2 year old had a very active social life, we were out every morning at different activities and most afternoons with friends, she wanted me to spend more "one to one" time with their 4 month old baby, which is quite difficult when you have a 2 year old and a 6 month old!!! We had to sit down and say that they have come into a share where the children already have a daily routine and if they didn't like it, they would have to find someone else, but they didn't want to pay for a nanny just for them so they stayed!!
Pay - One of my bosses was an accountant, so she did my pay, the families paid the same amount and she did the tax and NI for all of us, when extra children were added, I got £30 a week pay rise from the childs parents. I did get a slightly higher rate being in a nanny-share position than I would have had I worked for a single family and it was split down the middle. I had a kitty from both families of £20 a week and I kept that with me (I did account for it!) and if we didn't use it all up during the week I would carry it over to the following week and then when we had enough, we would spend it on a day trip to the zoo or something.