We have been doing it for 2 weeks, we noticed an improvement after the first night.
I put him to bed awake but sleepy, left him for 5 mins, went in led him down and shushed him then left him for 10 mins, did the same again, then left him for 15 mins, which is the longest I would leave him for, he went to sleep in the first 15 min cry.
After a few days he was settling straight away and sleeping for 8 hours in one stretch, For the past 5-6 nights he has slept straight through.
Hasn't upset him in the slightest, he cried out of anger more tha upset, now he is happier in the day, has 2 naps a day, we are all better off!
He even stayed at his Nans last night, I was a bit concerned that it would break the new routine but he slept through there too.
I think oyur Dd1 will be fine, like I say mine never wakes up when Ds2 cries, I think once they are properly asleep thats it!