I've not had 2 under 2, but pretty close. All my kids are 2 years apart, give or take a month or two. When my first was 22 months, my second came along (I guess that's technically 2 under 2) and I was so worried. My first had been a nightmare baby and was still having a lot of problems. We suspected autism based on a few different things she was doing (or not doing). So when #2
came along, I was scared. Not that he'd be a repeat of her, but that I couldn't handle a newborn/infant on top of my high maintenance daughter. She wasn't potty trained, she wasn't even really talking yet. She had out-of-body tantrums over the craziest things (not normal 2 year old tantrums).
So how did I do it? Haha! I have no idea! I just... did. The first couple months were hard. I had post partum depression and was recovering from a bad birth and unplanned c/s. But somehow, we did make it through. I don't remember it being horrific. I don't remember crying a lot or feeling out of control. I know the house wasn't always clean and I had to let go of some of my super-mom ideals. I spent a lot of time with a friend who had kids who were similar in age.
And around the time #2
started crawling, a wonderful thing happened. My daughter finally acknowledged him and started coming out of that secret little world she was living in. She started talking. She stopped lining up her toys. She potty-trained herself one night. It was like a miracle. I know this is off-topic, but I love remembering it and talking about it. It was like my son was sent just for her. Now they are best friends (and, sometimes, worst enemies). She is still a little "off", but only those closest to her would notice, and it's certainly no big deal.
So my advice is to not worry and to not spend too much time pre-planning. You'll spend some time in survival mode and go to bed pretty darn tired some nights. You might think you are going crazy from time to time, but it's so so worth it, especially when they get older. You will also have awesome moments when your heart melts and you will think you are the luckiest mom in the whole world
One more piece of advice- when you are faced with a situation where you pretty much have the choice of laughing or crying, just laugh. This usually involves some gigantic, horrific mess the kids have made while you are in bathroom or on the phone. Whether you laugh or cry, the mess will not go away and will have to be cleaned, so just laugh... it helps. But this is true even for one child.... It amazes me how much damage a child can do to a room in such a short amount of time!