I am in your situation. I think you just middle on through, like with everything! What you do changes as the children change.
When DC2 was newborn, I had him in a sling for the duration of DC1's bath & bed routine. After a while I started bathing him with her and was able to put him down in his cot while I did DD's bedtime - he often drifted off to sleep himself top, which was awesome!
Then he stopped going to sleep on his own, which was tricky for a while as I kept him up whilst I did DD's bedtime, but it wasn't v good as he was overtired, but we muddled on. Then, DD dropped her daytime sleep, which made it much easier because she was tired enough at the end of the day for me to put her to bed early first and then do him.
Now DS (18mo) is taking a long time to go to sleep with me in the room and DD (3.5) doesn't go to sleep as early as she used to, so we're back to a bit of muddling again. She goes to bed and listens to a story CD while I put DS down. It works 70% of the time OK, but at other time DS takes ages to go down and DD ends up coming and finding me, a bit fed up! No ideal, but I don't know another solution!
It works out ok. You make it work. And the children adapt to what you have to do.
Good luck with your new baby.