Some thoughts I had/have had in past (have worked in a few shares during my nanny career)...
Work out where you will be hosting - week about/month about/always at one house? if its not divided equally, make sure you work out costings for food between you. Its bad form to bitch to your nanny that you are providing all the nappies for the other family, sort it out in advance!
Make sure that your parenting approaches are similar - theres no use one of you wanting a nanny who is a firm disciplinarian and the other wanting someone who is laid back and fun. It just doesnt work!
Make sure you schedule in time for a regular review - more regularly at first, less frequently (but still necessary) as the share progresses... It helps to iron out the kinks, raise any issues or concerns, or just tell your nanny shes doing well!
Be aware that two is double the fun but also at times double the work - I went to one interview where they wanted me to go to the other house (the one that wasnt hosting) each week just to do the other childs washing so the parents didnt have to do any. They were several miles apart, I wasnt driving, and I was supposed to fit it in between all the activities they wanted me to do. Be aware that it also might take your nanny a little while to get into the swing of things - i know that with 3 or 4 children to look after, learning the ropes took me a lot longer than if i'd just had one baby.
Try it out for yourself, spend a day on your own looking after both/all the children you will expect your nanny to care for, and do all the jobs you'd like her to do - this way, you have an idea of what her day will be like and an expectation of what is reasonable to ask her to do.
Give a reasonable amount of kitty money - you don't have to go overboard by any stretch, but if you'd like her to do activities and outings (and most nannies will want to), make sure theres enough to cover it. Discuss with your nanny beforehand to find out what kind of outings she'd like to do, what activities you'd like her to do, and go from there. Remember that if theres two of them, and they're not twins/siblings, youre often shortchanged on the sibling discount (SIGH!) so make sure you allow for that too. Dont give her a tenner and then call and ask her to get a weeks worth of shopping.
If you write inclusive babysitting into your contract, remember that you are sharing the nanny - if you each want a nights babysitting a week (I personally go for one inclusive babysit per month per family, but all nanny agreements differ), thats around 8 nights a month that the nanny is giving up to babysit for you, so try not to make them all weekends or they won't get a chance to have a break!
Work out how youre going split the holiday and make sure you give the other family and nanny plenty of notice to take it as their holiday too if necessary, otherwise you may find you've taken all all your leave and your nanny hasnt had all her legal entitlement.
I'm sure I will think of more but thats a start!