I'm on mat leave no.2 at the moment, with our nanny staying on. It's definitely been awkward us both being at home, and I'm pretty sure she feels like I'm in her way a lot of the time. She also claims that DS1's behaviour is much worse when I'm around.
On the plus side DS1 has been able to keep up his routine, with play dates / toddler groups etc, which he'd have done much less of if it was just me, in the beginning anyway. And I've been able to dedicate more time to the baby than I would have otherwise.
One thing to watch out for - DS2 won't drink from a bottle, which has meant I can't really leave him with the nanny for longer than an hour or two at a time. And she can't ever take both boys out in case the baby gets hungry. Just something to be aware of - we might have been able to avoid it if we'd started him on bottles early enough, I'm not entirely sure.
In hindsight, I'd have liked to have the nanny part-time, to save on money, but also to have both boys to myself for at least some of the time - perhaps 3 days a week? We didn't broach the subject as we didn't feel it was fair for her to lose out on the income, but that would probably have been the 'right' balance for us.
And financially, we can just about manage to keep her on, but it has been more stressful than the first time round when I didn't have any outgoings apart from the odd coffee.
Sorry that was a bit stream of consciousness, but I'd say keep her on if you can easily afford it - you will really appreciate the help, but consider whether it needs to be full time or not. Good luck!