I can't answer the money question, but yes it's completely reasonable for the nanny who primarily takes care of the baby to take care of the older child during snow days, school vacation days, when older child is mildly ill and can't go to preschool, etc. And, yes, your nanny can pick up/drop off older one with the baby in car seat or stroller (depending on distance).
This is what a nanny does - makes your life easier and cares for the children - and it's why many people opt for the full time nanny when they have 2 or more children, because it makes their lives so much easier.
You will need to build this into the contract and expectations, and find a way for your nanny and older child to bond so they aren't strangers to each other.
the problem with using a nanny share and doing the above is that it might be hard to suddenly bring the older child in for all those times, especially when it's at the other baby's house, esp. if that other baby is an only because there won't be any toys and activities for the 3 yr old. If the share is at your house, you could discuss with other parent having your older at home for those reasons, but they might not be comfortable with idea of sometimes 3 in the share. And taking 2 babies around to pick up/drop off at preschool would be a bear and a half, not to mention the 3 carseats your car or nanny's car would have to fit. Ugh.