We had our 2 (girls) close together - one and a half years apart - but it wasn't intentional. We 'tried for' DD1, and I got pregnant very quickly. Then I went back on the pill shortly after with no thoughts about another one (at that point.) Then I was ill a couple of times with the trots, and genuinely didn't think anything of it, and the pill failed.
And so a year and a half after DD1, along came DD2.
I think I was 3 months pregnant with DD2 before I knew I was pregnant, because my periods were all over the place anyway. We weren't prepared for 2 under 2, but the Universe had other ideas!
Anyway, so we had 2 under 2. Didn't plan DD2, but she was very much wanted and welcomed - and as loved as DD1, and having 2 close together is the best thing we have ever done! Tbh, it has been fine!
They both had their own bedroom, so if one woke the other didn't, and DD1 was sleeping through the night by the time DD2 came along, so we weren't getting up to 2 of them all night IYSWIM.
I don't think it would have been any different if we had had a 5 year gap or more. Did spend more (I think,) as there were not that many 'hand me downs.' And we had to buy 2 of the same thing for Christmases (like they both wanted 2 of every Bratz doll thing, or Barbie thing!) And we had to buy 2 school uniforms, (in fact, 2 of everything at school, as they were both mostly at the same school at the same time!) But you would have to do that anyway for anything less than a 4 year gap I think.
My 2 were actually pretty good kids, well behaved and played together nicely, and rarely gave me and DH a hard time. DH and I loved having 2 little girls SO much.
Going on holiday as a family of 4, people always made a fuss of our 2 girls and said how well behaved they were.
Then they hit 13-14, and the shit hit the fan. Teenage years are such a hard time, for the teens AND for the parents. And teenage girls are more difficult to deal with that teenage boys. (I think!) Quite a few teachers told me this.
So yeah, we struggled with them for 3 years or so, as they were rebellious and talked back quite a bit, and argued and fought and fell out with different girls. But by 16-17 they had grown out of it. Now, around a decade later, they are intelligent, independent, feisty, kind, funny, loving young women, who both have successful careers, and have settled down with a partner (and both have their own home.)