I had two under two (17 month age gap). It was a nightmare to begin with but I think I made it more difficult for myself because I had depression and we moved house when dd was 6 weeks old.
Dd was a difficult baby in that she cried a lot, had reflux and always seemed to catch every cold and chest infection going.
This difficult stage seemed to last forever but really, by the time dd was 6 months old, fully weaned and both dc fell into a routine of shared naps and bedtimes, it was so much easier and continued to do so as she reached every milestone and inched her way to greater independence - I wasn't having to do a billion things at once and trying to pull myself in two or three trying to sort out myself, both dc and the house.
They are now 6 and nearly 5 and are really close. They do argue and bicker of course but play together lovely, entertain each other and look after each other. It's lovely to watch and listen to them interact (the other day, they were building a castle with the Lego and ds said to his sister, "Teamwork..."
Dd replied, "Makes the dream work!" I think they got it from some TV programme they'd been watching).
I haven't really got any tips other than to lower your standards re. housework, don't stress about your older child watching more TV than before, expect and be prepared for your 2nd child to cry more than your first simply because you haven't got the time to run to them as soon as they need you to, if they both need help go to your toddler first because they're usually easier to sort out, don't feel bad about using ready meals and quick teas because any food is better than none, try to include your toddler as much as possible in the care for the baby so they don't feel left out and just try as much as you can to relax!
Good luck and congratulations on your pregnancy!