I have BTDT with DS and in a few weeks time when DH goes back to wor we will be doing it again with DD.
I did almost exactly what you did, and I found that ds needed no time to adjust, in fact when the new nanny came over seemed almost relieved to have a new person to babble to and make eyes at, like he was sick of the sight of me. It hurt my pride a little, but then I went and got a manicure for the first time in nearly a year and I got over it.
Plus he was really happy to see me when I got home.
I think it depends on your kids, and YOU have TWO so surely that will take an nanny some getting used to, so what you are doing sounds perfect.
Is she going to live with you? If she is I would expect her to either help make dinner and breakfast, or help look after the twins while you do, and when she is alone with them I would be careful to discuss certain family value things, and dietary things...like will you be pumping, or can she make formula or both? Is she allowed to give them food, if so, what kinds and when? How many naps do you want them to have and when (I had a nanny with DS who would put him down for a nap almost as soon as I would leave and let him sleep for four hours, and then again right before I would come home at 5pm so he would wake up at 7 full of energy and ready to play and having had about 6 of his usual 13 hours of sleep.) Do you expect her to do light cleaning while they are napping, tidying of toys and diapering area, dishes, that sort of thing?
The clearer her duties and your expectations are, the better, IME. I doubt she will feel you are being condescending or bossy...you're her boss. You're supposed to tell her precisely how to do the job. and it's way easier than finding out six months down the line when your kids and you are attached that she keeps them calm by giving them a wee bit of grandpa's cold medicine, or that she makes them smile upon her arrival by sneaking them lollipops and coke when you are gone. It is so much harder to tell someone off than it is to tell them not to do things in the first place...plus then when they do things you didn't want them to do, you can have grounds for letting them go.
Oh and NOTHING is common sense when it comes to child rearing, there are ALL sorts, so take nothing for granted. Like "oh, surely no one would give a baby Coke!" or "Now who in the world would give a baby whisky mixed with honey?" or "why would anyone give my baby a pacifier dipped in jam?" or "Now no one would give a tiny baby a chocolate chip cookie, you'd have to be stupid to do that!" or "why in the world would anyone leave a six month old to soothe herself when she is scared or hurt?"
You'd be surprised what seemingly perfectly reasonable and ordinary people are capable of when it comes to childcare. Better not to ask and just make a list of don'ts now.
My only other advice is that IF your kids are as easy going and happy to be left with the nanny as my son was and it seems DD probably will be (based on a few experiences with a sitter), don't take it to heart. It doesn't mean anything. It's like when you find a new friend to flirt with; it doesn't mean you love your partner less, you just have fun feeling new and shiny again for a little while. That's all...the new caretaker smell wears off eventually and then they'll be all the more in love with you.