Let daddy do it without stepping in. This is really difficult and we went through it when my son was younger, but I was taking a night course and daddy needed to do bed time two nights a week.
First suggestion, let daddy and toddler have their "own" routine that is special for them. I always read to my son for 10 minutes before he goes to sleep, but my husband cuddles and watches a few minutes of t.v. with him. It's "their thing" and my son has learned that if momm is out-of-town or working then he "gets" to watch Dora with daddy. Bonus.
Second, leave the house for an hour so that neither of them can come to you. Let your husband know that you completely trust his ability to do this, but you don't think you could stay out of it while they figure this out, so you are going to use that time to hit the grocery story or something else that makes you "unavailable".
Third, try to create more independence in your older child so that he can get himself to sleep if needed. Are there things he can help with? Can the "big boys" brush their teeth together?
Fourth (and final)... try not to respond to his every need so quickly! If you start requiring him to "wait" now he won't associate that word with the new baby.
Good luck!