1) Each family pays taxes on their portion of her wages. I recieve 2 paychecks each pay period, and 2 W-2's at the end of each year. Tax wise it is 2 jobs.
2) In each share that I have worked, my salary has been split equally between families, as there are trade offs to being either host or non host family. You can share hosting in any manner that you like, or designate one host family. As a nanny, I abhor switching houses constantly and would not consider such a share again.
3) Non hosting family typically sends their own meals and snacks, unless I go shopping to prepare meals, in which I give them the receipt and they split the cost.
4) I recieve 2 weeks of my own vacation, of which I am free to schedule as I please. My families typically take vacation in addition to my own, but they are under no obligation to coordinate it with myself or the other family. I have guaranteed hours, so I recieve my full weekly salary regardless of if both children are present for the full schedule. It isn't vacation for me if I still have to be at work with the other family's child.