We're in a share, and for the first year or two we seemed to be at our house more and therefore scoffed more yoghurts, burnt through more wipes and generated more chaos. We reasoned that the plus side of this was that it was our children who had the comfort of a "home match" most days. We felt that we couldn't really put a price on that.
When there is an age gap between children, as we had, we stayed at ours when DS was small enough for a kip in his own cot to be real advantage, and now that he is older, he plays at the other house so that their baby can be at home. And he is busy eating their yoghurts and using their paints. Over the four years or so, I think that it will all have evened out.
Try not to see the give and take in purely weekly terms, but in longer measurable periods, i.e. a term, three months, etc. Then you can look at the broader routine and less at the minutae of the day to day.
In our kitty each family has its own purse, so we don't subsidise on days when she is just with the other family, and vice versa. Also, they have more hours than we do, as I only work part time.
We talk and email regularly, sit down and have a full meeting each July to plan the following year - we are both MB teachers - and ensure that we write everything down, i.e. who will be doing which regular activities one each day. We look at the longer picture here, too. This year my DS has been following the older child around somewhat, but next year, in his last pre-school year, he will get to do more activities himself while the toddler gets to watch him do gym, as the older two will be at school.
We ensure that any issues over hours or possible tangles when we need our nanny to be at opposite ends of town at the same time are not discussed through the nanny so that she isn't put into a difficult position.
She's now worked for me since DD (now 5.5) was nine months old, and this is the second family that we have shared her with. We have been really, really, happy with her and the arrangements, albeit with an acceptance that we have to make some compromises.