Aw, I SO feel your pain!
We had similar issues when P was a baby. Y'know there were many nights when I just did what you are doing - juggled the two of them for what felt like an eternity.
I found I couldn't bring P into N's room because that would just make her jealous and even more upset (I imagine if your DS is still bfeeding then bf DD in front of him will not help terribly much, although I guess it's worth a try to see if your presence there even if she's there too is enough).
I used to try and time P's last nap of the day to commence right before N's bedtime although there were only a couple of occasions where this really worked. These days P is ok at being left in bed with a book and the hall light on. She often entertains herself with it (usually by banging it around rather than actually looking at it, lol!) and this entertains her enough til she is tired enough to fall asleep. But she's a good year older than your DS... (because at 5yo Nat often still requires my presence with her as she falls asleep).
Hugs - hope you work out a groove soon.