We did it.
Whether you or your partner got pregnant in 2 years, it would still be raising babies forever, right? The order seems irrelevant to me.
I always said it would be dumb to get pregnant again once I was sleeping through the night again, but I did it anyway.
We have a 9 year old, an almost 7 year old, a 7 month old and a 6 week old. The older two are from a previous relationship and spend half their time at each house.
Sara started trying the month after I got pregnant. It was hard for her trying while living with a prego. I have pretty easy pregnancies, but being pregnant in my 30s was much harder than being pregnant in my 20s. No real problems beyond heavy, tired, achy, cranky. Sara got pregnant as I hit my third trimester. Sara's pregnancy sucked. Early pregnancy exhaustion and a partner in labor overnight wasn't fun. She was medicated for morning sickness into her third trimester (and raising a newborn). She had mid-pregnancy bleeding (while raising a baby). Heartburn, risk of preterm labor (as that baby passed the 20 pound mark), etc. Post partum hormones, PPD.
Having a 6 month old and new baby is a balancing act. Trying to keep a mobile infant from clobbering, scratching, smacking, biting and lifting his brother's head has been a trick. The tiny baby is the least demanding member of our household, I think (Sara might feel differently, by my measure, the 7 month old is the most demanding, screaming for boobie every time the baby cries. I joke that he took the idea of survival of the fittest as a challenge). We wash diapers daily, on top of our normal laundry. Because the 7 month old can now nurse with us both, he occasionally refuses the bottle, but Sara doesn't make that much extra milk. Because the tiny baby can nurse with me, he expects to, but Sara would get engorged if I nursed him willy nilly. (so it's an advantage, but not perfect right now)
The 6 year old had some clingyness right after the 7 month old appeared, it came back some after the new guy arrived. She also really had a hard time with so many hormonal women in the house.
The 9 year old developed insomnia and anxiety after the newest guy showed up. We're still working on that one.
To say that we are busy is putting it lightly. I consider driving to be a break right now. Work is kind of like vacation. I am one of the most easygoing people I know, so I deal with it ok. Just busy, not so much frustrated with the extra work, it was pretty expected. I'm getting pretty good at tandem nursing, tandem baby wearing, keeping everyone fed, clean, engaged. Safe is hard with a mobile baby. He climbs onto and then throws himself off everything.
If you think there could be any difference in how they are treated by grandparents, well, you'll find out really quickly (Sara is Jewish and the ceremonies were more heavily pushed with the child she carried, which hurt her feelings because she felt like it showed that her parents didn't view the 7 month old as her "real" son).
If I could do it differently, I would have had Sara, with her wildcard of a pregnancy go first, because not only did I have an easier pregnancy, I had a significantly easier recovery. I would have tried to have them closer together, so they could be closer in size and development. Otherwise more of a year between them, because 1 year olds are a little more able to understand and walk (of course, not all 6 month olds crawl, it was a gamble).
I am pretty happy, all these negatives are just things that are drawbacks you may not have thought of. I would still have done it, knowing what I know now.