Op - I had a shared care nanny 4 days a week for just over a year as I was trying to study at home. It did work out quite well most of hte time - as she didn't need to be highly qualified I hired one of the staff from DS1's nursery who didn't have nannying experience and included a few light housekeeping duties as well.
It gave me the opportunity to do 1 on 1 things with the DC, and made other things easier. Unlike Karoleann I didn't work it so that one of us was always out of the house. There were a lot of times that we were all in the house at the same time, and even did outings together. 2 adults taking the DC out and about certainly made life easier for me!!!! I think what helped make it work was that the DSs knew who was 'in charge' at any given time as it were. So if I wanted to 'run' lunch, I made that clear to the nanny and I took control. Otherwise I would let the nanny 'run' lunch and I would join them, but backing her up if they played up and she had to discipline them.
I guess generally if the nanny was there, she was in charge of them unless I said otherwise, and on outings I would generally be in charge - but obviously you need someone fairly easy going because there will always be times that you step on each others toes, you just have to work your way through it (and not be precious, let the nanny do things a little differently to how you would!)
I also found 4 days a bit much for me in the end as the DS2 got a bit older.
Also, because I was around a lot I chose someone without a driver's licence thinking it wouldn't matter, but in the end it did start to matter as I became resentful that I had to always interrupt what I was doing to do the nursery runs, etc. I had asked her - and she agreed - to get her driver's licence as soon as possible when we first talked to her about the position. It didn't happen and we were stalled in that routine without being able to change it so think carefully about how your needs might alter and try to choose someone who would meet any altered needs.
Also as Karoleann said, as you will be around a lot - definitely get someone that you get along with. That is VERY important. Even now, having stopped using my nanny I still use her for occasional babysitting and we catch up every now and then because I liked her as a person and loved the way she interacted with my DC.