We’ve just got an out n about double nipper and I absolutely love it! So light and easy to push, plus fits through a standard door! You can get carrycots for them or newborn inserts but honestly it’s a dream. We went from a tandem and the difference is incredible. The bugaboo donkey is really popular but is pretty pricey and quite bulky/heavy in my experience, but does go down to a single which might be good for when your older one is walking. I’ve also heard great things about the mountain buggy and I know you can get carrycot attachments for those too.
City mini jogger 2 is by far the best buggy I have used including single buggies
I think a fan favourite for double buggies is the bugaboo donkey as it can be single or double. I don’t have one myself but I see them EVERYWHERE