I need help!
I have a wonderful 2.11 dd who has an established bedtime routine.. (times are approx)
6pm dinner finished
6 - 6.30 quiet play (puzzles, bead threading, etc.), cbeebies bedtime hour, stories
6.30 - 6.50 bathroom to get ready for bed. We bath every other night but it doesn't take that long.
6.50 - 7 into bedroom to get dressed for bed. Story and song
7pm Lights off and door closed.
I also have a 10 week old ds who is definitely starting to need a proper bedtime (instead of napping until the 10pm feed when I go to bed) and can tell the difference between day and night (or is starting to).
I am conscious of the fact that I say goodnight to my dd with ds on me in the sling. Sometimes he's asleep, sometimes awake. Sometimes screaming and ruining a peaceful bedtime for my dd 
I should add that I usually am alone for bedtime. Dh doesn't get home until 6.55 at the earliest, which means that occasionally he gets here just in time to read dd a bedtime story or sing her a song.
I am a self-confessed control freak and like routine. This juggling of bedtime is stressing me out and I just can't figure out the logistics. We have had a couple of occasions where I have managed to put ds down and then take my dd to bed, but I just can't seem to organise myself properly. I'm also scared of trying in case it doesn't work and I get frustrated.
Please share your pearls of wisdom - actual logistical help required!
p.s: Not sure whether relevant, but dd's room is in the loft, so we're not all on one level making it a bit more tricky.