In my experience in this type of set up, you need to come up with rate that is fair for the full 40 hours. The family using less hours always ALWAYS ends up taking advantage of those extra hours. Either they constantly want to bank or shift hours, or they don't respect their end time with the nanny and are often late because they know the nanny works for an extra hour, or they show up directly at their end time and feel entitled to a 15-30 minute daily check-in because nanny has nowhere to go. I would say to offer a slightly lower hourly rate, but be prepared to pay it for the full schedule, with the right to use those hours in any way that you like.

It depends on the nanny.
Will the other family ever need more than 32 hours? Is there a way you could meet halfway?

I am a nanny in a share. One family needs me 9 hrs a day and the other 8 hrs (usually.) I charge the share rate for the 8, then regular single family rate (not double share rate) for the remaining one.

We are starting a nanny share with another family. They only need 32 hours/week while we need 40 hours. Would we each pay the share rate (eg $10/hr for each family) for the 32 hours and then we pay the one child (eg $15/hr) for the other 8? I realize most nannies want a guaranteed 40 hours but I also don't want to have to pay a two child (in this case $20/hr) rate for the remaining 8 hours just so the nanny received the full $20/hr @ 40 hours as that doesn't seem entirely fair to us. Worst case, we will absorb the extra but just wondered how others have handled this.