I love my out and about nipper - narrower than it looks and goes through my doorway. I've had it from birth and it's been perfect. I considered a tandem as a 2nd buggy but was advised I would find it heavy with 2 in, especially as I was so used to how light the nipper is.
If space was a big issue I'd probably go for a Phil and Ted as at 2 dd1 was walking lots - although dtds were prem and at 14 months aren't walking yet so might be different with them.
Can't believe you made it to nearly two years with the peach, my two are just nearly one and I am starting to hate my icandy as its just so hard to get lower twin in and out. She grabs the bar and swings from it (j know its best to take top seat off but not always possible)
I bought an Obaby twin sport thing for holidays but am using it nearly every day now as they live sitting next to each other and both having a proper unobscured view. It's slightly narrower than a standard door, we also live in a Victorian terrace and it just about scrapes in. Not much harder to push but my two are much smaller than yours.
The Out n About Nipper will fit through most doorways - its much narrower than you'd expect. Its also a doddle to push even with 2 hefty toddlers. I loved mine. The only downside (unless they've changed the design recently) is that you can't adjust the seats so they're completely upright, they will only go so far which means the children are very slightly reclined. Mine hated that as they got older, and we had to put cushions behind them so they could sit completely upright and look around.