17 months apart here.
Honestly? It was the hardest year of my life. Seriously. Put a huge strain on my marriage. Sleep deprivation makes everything more challenging and toddlers are plenty challenging under the best of circumstances. Very little time for yourself and even less for your marriage so my #1 piece of advice is to attempt a game plan to carve out some "me" time and some "couple" time utilizing sitters if you can. Even a once a month date night with a rule that you can NOT discuss the children will go a loooong way in helping to fortify your relationship as you navigate these rough waters.
Sit down with DH and strategize how you are going to come at this challenge. Without candid conversations about division of duty there can be a lot of resentment. Chances are that even with clear communication about who's gonna do what there will still be days when both of you will feel like you're doing everything.
Advice: "This too shall pass". This was my mantra. It won't last forever. The difficulty of having 2 so dependent upon you at the same time is FAR outweighed by how amazingly cool and easy it is as they get older. Video as much as you can. I find that I don't remember much of this time in our lives (see sleep deprivation note above) and I love being able to look back to remember. The kids ADORE seeing themselves together at super young ages too. Get in the picture. It doesn't matter if you look like crap and you haven't lost the baby weight. They want to see their family and they love you regardless of how you think you look.
Other advice: "No, you are not permanently damaging either of your children by loving or tending to the other while they cry/wait/pout, etc" It's not a bad thing to learn early that you are not the center of the universe but that doesn't mean that you are not important. They will not remember this time in their lives. They will not remember a time when their sibling didn't exist. Survive the best you can, love them the best you can and have faith that it will be enough even when it feels like it's not.
Even more advice: Everything will feel crazy at first but before you know it you'll have a routine in place and things that once seemed impossible (how the hell do you get them both in and out of the car without someone running into the street and getting killed?) will be a snap. And... as soon as you get it all figured out it will all change and you'll be back to having to figure it all out again.
BEST advice: Come here with questions, especially about logistics. No need to reinvent the wheel. These women are a wonderful support and have lots of insight mostly born of trial and error.
Congrats on your pregnancy and welcome to the journey!! It's the craziest but most amazing and rewarding thing you can imagine.