Let me share how things are going here. I have a 25 month old and a 3 month old.
Before dd3 was born, I had to lay down with dd2 in her bed (she was moved to a big girl bed at 14 months because she was climbing out of her crib and getting hurt,) until she fell asleep. Once asleep, I slipped out of the room. This was for naps and bed for the night.
I tried several times while pg to try to get her to go to bed on her own. I tried using Supernanny style methods where you basically just keep putting them back in bed without saying a word. Being pg made that impossible, I simply did not have the energy. Once dd3 was born, I struggled for 2 months with having to force the 2 year old to wait until DD3 was asleep before I could get DD2 to bed. Thankfully DD3 has been a mellow baby, in all the ways DD2 never was. She's one that I can, for the most part, set down in a swing or cradle or playpen for a short time without her getting upset, so if I couldn't get her down for a nap before DD2 just lost it, I could at least let her rest for a bit while I worked on DD2. And, if I had her asleep and DD2 was being difficult, I would then switch off, letting DD2 play while I rocked with DD3. And yeah, there were many days that I spent most of the day just switching off between the two trying to get or keep one down for their naps. Thankfully at night, I had a 15 year old to help out those nights that DH was in class.
However, when my now 25month old had her second birthday, everything changed for the better. Not because she turned 2, but because she received her first lovey. We watch tv in our house, lots of Playhouse Disney on in the background while doing playdough or while I am nursing the baby and need to keep DD2 occupied, etc. And Mickey Mouse has become her FAVORITE character ever. That lent itself to lots of Mickcy Mouse gifts, including a super cute stuffed "rag style" Mickey. Basically just a super simple, slightly abstract stuffed animal Mickey. She immediately became attached.
Within 2 days, DD2 was going to sleep on her own, in her own bed, without anyone laying with her, without even having to return her to bed once! It was as if Mickey took our place in reassuring her that it was ok to be in bed alone. Or rather that with Mickey, she didn't feel alone.
Also, we have developed a great routine that I think helps as well. It took a while to get to something comfortable, without creating a dependance on something else.
SO, we start around 7 or 7:30. I change DD3 diaper and into "jammies," and swaddler her. I nurse her while DD2 gets to watch DVR'd Mickey Mouse Clubhouse or Special Agent Oso. Sometimes she wants me to read to her while I nurse but that's tough because she tends to want to climb on top of DD3. Once I am done nursing DD3, I lay her down in her cradle. Then DD2 and I go around and pick up and clean up the playroom and family room, DD1 (the teen) is in charge of the kitchen/dining area. Then I change DD2 diaper, and get her into jammies. She gives DD1 and DH (if he's home) a kiss, then we go upstairs and brush teeth. I brush mine while she does hers, then I take my turn brushing her teeth again. Then we go get her do-dah (paci) from it's special spot, she only uses it when sleeping. Then she climbs into bed, I give her Mickey and her other new lovie Raeanna Pink Bear (Raeanna is DD2's name, the bear is a pink beanie baby bear.) Then she snuggles into her warm comforter and also a quilt DH's grandmother made herI give her a kiss, tell her night night and to stay in bed, and she tells me night (which ends up sounding very much like "DIE" in her language :D ) And she rolls over and goes to sleep.
Naps are actually done exactly the same way.