I kept my FT nanny on during my maternity leave. From the outset I knew that the most I would be taking was 6 months off plus accrued holiday and it made far more sense to keep her than risk losing her and trying to hire a replacement.
What it did enable me to do was to return to work part time, working from home after 12 weeks. I was on Maternity Allowance as I fell pregnant just before starting a new job. Rubbish timing as missed their 6 months enhanced package by 2 weeks. But I digress. Returning PT at 12 weeks certainly removed the financial stress and made returning to work easier I think in the long run.
DD2 was mixed fed due to feeding difficulties from birth so it was relatively straight forward for our nanny to do a mid morning or afternoon feed if they were out and about. Also, she had never worked with a newborn before [as most people will take at least 6 months off] so had always started around the weaning stage so a phased handover gave her a lot more confidence dealing with a tiny. I disagree that your au-pair won't cope - babies take practice for everyone, it's more about interest and aptitude in the first place. Coping with two all day though, that's a proper nanny role. If she is definitely returning home that's one thing but if she's great with your DD1, and you will take a reasonable period of time off, you could look at her doing childcare quals and becoming a live in nanny. You would obviously have to be far more hands on and directive than you would with an experienced nanny though.
Issues - because our nanny was FT life went on as normal for DD1, 2.4 at the time. It became apparent that she felt very pushed out as they got on with their normal routine but Mum was suddenly around all day giving cuddles [bfing] on the sofa to her new sister. In hindsight, while it was obviously fabulous for me to have someone around FT, it might have caused less resentment if she'd spent more time just hanging out on the sofa during feeds watching Cbeebies as if I'd been a normal SAHM just getting on with it. DD2 was a very poor feeder with all the stresses that came with it. I was expressing every feed for 6-8 weeks and life basically revolved around getting some meat on her bones. I wish I'd just given her a bit more formula, handed her over for an hour or two and spent some alone time with DD1. It didn't help that I was pretty uncomfortable for 12 weeks after the birth so even a short walk to the park filled me with dread.
Going back to work FT at 6 months was a doddle, I had no concerns about our nanny and DD2. They had been going out as a group for three months by then and had found their own routine.
From our nanny's perspective I think the biggest adjustment was juggling the two ages. DD1 had dropped her nap and they were out and about a lot often without a buggy jumping on and off buses etc; and suddenly she was trying to entertain an older child along with establishing a routine for a baby. She was a big GF fan and so had to be back for lunchtime naps yada yada. Rod for her own back but I left her to it and to be fair to her she did a great job. We were very sorry to see her go in the end.