A.F.
I have two kids, and I have also been through a nanny share experience with my first son. Here are my thoughts:
I think a nanny share is great, and I do think that you need to schedule meetings either with the other mom or the family from time to time to touch base, not just try and get in a word during drop off or pick up. I did this and we would talk about our schedules, changes in eating habbits, naps, anything we wanted to address with the nanny, etc. It was a huge savings to us, but the best part is that as each child approached the age of 10-12 months, they started to become really close and were the best of friends. My son, four years later is still really close to the child he was in a nanny share with even though they have now moved.
I think that some suggestions to the nanny and the other mom can be really helpful. The two suggestions are that: 1) the nanny needs to wash her hands frequently, as well as the baby's hands, and the other mom could probably benefit from more handwashing at home. It is the best way to prevent illness. I know it sounds compulsive or something, but when one of my children gets a cold, I have been able to prevent the other from getting it simply by frequent handwashing.
2) I think another important thing to stress is that you and the other mom have to agree on full disclosure about your child's health. Even if a child does not have a fever and they are feeling well enough to be with the nanny, they can still be contagious. So, she should have to tell you if he has a cold, or if he is just producing more fluid due to teething, etc.