I dislike shares but will add…
Make sure you’re on the same page with the other family regarding guaranteed hours, when kids are sick, be respectful of her time- ie…being on time for end of day pick up (plan to get home a few mins early to get a recap so that she’s out the door on time), paying on time, discuss expectations of her when kids are napping. Assuming you’ll have a double stroller- have it set up for her especially if you have stairs, leaving petty cash in case if she wants to buy them a treat while out, respect her working space and don’t hover if you’re a WFH (no intrusive grandparents hanging around either), having a clear plan in case one or both of the families gets pregnant again or backs out of the share (Nannies depend on their money).