• The Nanny.  What type of nanny are you comfortable with? What age range, tribe, level of experience and/or educational level are you willing to consider? Must she come with extra skills (hair braiding, housekeeping, cooking)?
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• Compensation. Document nanny's total compensation and how this total is calculated between each family. Discuss frequency and tax treatment - if any. How will you coordinate paying the nanny?
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• Payment to nanny for times when one or both families don't need the nanny on a scheduled day(s). The best practice is that nanny should be paid by all parties for all scheduled days that she is available to work. This includes the Friday when one family decides to get an early start to the weekend or the week a family is at the beach.
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• Notice/severance agreement between families when one wants to leave the share. What will happen in the event of one family need to end the share? (e.g., loss of a job, moving, etc). If family A decides to leave the share, there should be either a notice period or pay to the nanny in lieu of notice provision that allows nanny and family B to make arrangements to either find another family or end the share completely. Two to four weeks notice or four weeks pay in lieu of notice by the departing family allows the remaining parties the time to make necessary adjustments.
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• Nanny's notice requirements. When nanny leaves, the notice requirement should not exceed the notice that parents are willing to agree to in item 3 above. Two weeks is a minimum - four weeks by all parties is more reasonable.
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• Coordination of paid time off. How will nanny's sick time be handled? Vacations? Will nanny have at least one week of vacation time at her choosing? What is the notice provision for vacation schedules? When a nanny is sick, how early do the parents require notice and who is notified?
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• Overtime compensation, especially when only requested by one party. If one family requests 3 hours of overtime in a week, how will it be compensated? What about off-hours childcare?
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• The nanny's workplace. Discuss whose home the nanny will work in, or whether she'll alternate. How will you split the time at each family's home? If your nanny will be working at one household, factor in the extra expense for the host family when you're dividing up the costs. If family A hosts the share but takes a week vacation, does the share relocate to family B's home? Is this geographically friendly to the nanny?
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• Childcare duties and expectations. What are the expectations regarding activities, naps, feeding? How is a sick child handled? Iron out these details in advance. Look forward - when the babies are on table food what will it be and who prepares it? If family A is liberal on candy consumption and family B wants their child fed organic always, how can that be managed? Will the families provide petty cash to the nanny? How will/can it be spent? Will she be expected to provide extra services to the home while children are in her care? How will she manage that?
• Who will be responsible for transporting children from one home to the other? Will nanny take the children to activities? Whose car? Are car seats needed?
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• Parenting styles. This needs to be discussed upfront. It is generally easier for a nanny if all parents are on the same page Nanny, may struggle in a situation where families have dramatically different expectations on this front.
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• How will you handle communication with the nanny? Will, there be one point person, or will both families speak to the nanny about changes together? (Any policy directives for the nanny should be coordinated between the families so the nanny is not getting conflicting messages. It is less important that there be ONE PERSON to speak with the nanny than ONE VOICE, agreed upon by both families.)
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• Care area and consumables within the home. Will there be a designated area for childcare? Will additional childcare equipment have to be purchased in each home? How will you coordinate weekly expenses and supplies, including diapers, wipes, formula, food, etc? Will family A have to stock up on supplies such as wipes, diapers or supplies for meal preparation, etc,  when care is provided at their home and vice versa or will each family bring their own supply to the venue of care?

 Remember, details count. Putting it down in writing gives everyone a frame of reference, avoids misunderstandings and ambiguity, and generally just makes the share go more smoothly for all.

All that's left to do is to interview candidates and hire your agreed best option.