My kids are either 2 minutes or 17 months apart, depending on who you're talking about.
I think that would have been excellent spacing for singletons (based on someone with my interests and strengths, I'm not a real baby person, so I didn't want to do the baby thing extended out over years), but I've got to tell you...if I had known round two would be twins I probably would have waited a few years if I could guarantee getting the same kiddos.
I think any spacing could be bad for some kid combos, or positive for others. What I've noticed in observing a variety of families with lots of different numbers and spacing of kids is that being happy with the spacing seems to be less dependent on the actual spacing and MORE on the temperment/dynamic of the folks involved. People whose kids don't get along might blame it on spacing, people whose kids are very tempermentally compatable might attribute it to spacing. I don't doubt that spacing can influence that, but I don't think to a huge degree.
I actually feel that to some degree the spacing worked out the best for my daughter. Once the boys had been here 2 weeks, it was like they'd been there forever as far as she was concerned, and a lot of the asinine/inane commentary that people make about/to siblings of baby twins sailed right over her head. Once she was old enough to feel that kind of thing more, the boys were older and less dependent so I could shift more individual focus on her. And she didn't get caught up in the worry/drama like an older child might have, during our very stressful high-risk pregnancy--she just nursed through long appointments, got spoiled by the perinatology office staff, and could adapt quickly before she started needing things to be more routine. But again, that's less of a spacing thing and more of an individual thing--that's where she was at and what she was like at that time.
And my kids all have very compatable personalities, which I attribute to luck and genes rather than spacing, since I know people with similar spacing but kids that go together like oil and water.
I guess I'm being longwinded in saying why I think there is no perfect or forbidden spacing, that you need to be flexible/adaptable no matter what spacing you end up with because children and their personalities don't always go how we think they will/should, and as long as you maintain that flexibility and a sense of humor then really you should be able to handle just about any spacing even if it's not exactly as you wanted it to be or thought it would be.
