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The total rate the nanny would charge would depend on the total number of kids. Each child will add a small additional fee (but not double the original fee) So let's say the going rate for one kid is $18/hr. And each additional kid adds $3 an hour. I know you have twins, and let's assume the other family has one child. So now you're at a total of $24/hr with you paying 2/3rds ($16/hr) and the other family paying 1/3rd ($8/hr).

In my experience, a share works one of two ways: (1) you share a nanny with a family and each only use her part time, and never at the same time (you have nanny MWF, other family has her TTh, for example). You each pay $18/hour for your hours. (2) You and another family share a nanny for the same hours, so the nanny is taking care of 2+ kids from two different families at the same time. You pay a TOTAL amount per hour that is more than $18/hour, but not double (nanny share rates where I live are typically $20-26/hour for 45-50hours/week). The families split that rate, usually evenly, but sometimes not (for example, if one family hosts 100%, they might pay slightly less because they are incurring wear and tear, higher utilities, etc.).

In my area, nanny shares like in situation #2 are almost always just two kids (close in age), so it's like taking care of twins. The only situation I know where the nanny watches 3 kids is a case where there was an established 2-kid, 2-family share and one of the families had a second kid, and the nanny was willing to stay with the share (for a raise that the family with two kids paid).

Other things to consider - who will host? If you split hosting, how often do you switch houses? How to do handle things like a double stroller, two high chairs, two places to sleep? What do you do when one kid is sick?